These French Onion Mashed Potatoes are rich, creamy, and full of flavor, with golden caramelized onions swirled into buttery mashed potatoes. Serve them the same day or make them up to two days ahead. They reheat beautifully and retain that just‑made creaminess.

Top view of French onion mashed potatoes swirled with caramelized onions and garnished with fresh thyme.

A quick note before you begin

This recipe is simple to make, but it works a little differently as you prepare it compared to regular mashed potatoes. I’ve added a few extra details to help you get the best result.

The ingredient notes, step-by-step photos, reheating and storage tips, and the video all show exactly what to expect before you reach the full recipe card below.

Taking a minute to read through will make the process feel much easier, especially if you’re making this ahead.

If anything feels unclear, feel free to leave a comment. I’m always happy to help before you begin.

How this Mash holds up in the fridge

Regular mashed potatoes usually firm up and take on a slightly uneven look once they’re chilled, even after just one day in the fridge.

This recipe works differently.
The combination of butter, sour cream, and dairy keeps the texture soft and smooth, even when chilled for up to two days. When reheating, adding 2–3 tablespoons of milk or cream simply restores the warm, just-made texture.

Ingredient notes and substitutions

Listed below are notes for the main ingredients. You’ll find all other details and exact amounts in the recipe card at the end of this post.

Onions: Yellow or white onions work best because they caramelize evenly and stay soft. Red onions also work but turn darker once cooked.
Butter: Used twice — to caramelize the onions and again in the mash. Both salted and unsalted work; just adjust salt at the end.
Olive Oil: or use canola or any neutral oil It helps the onions brown evenly.
Thyme: Fresh or dried thyme pairs well with the caramelized onions. 1 teaspoon of dried Italian seasoning also works.
Heavy Cream: May be labeled double cream (UK) or thickened/whipping cream (Australia). Anything around 30–36% fat works the same.
Whole milk: Keeps the mash balanced so it’s silky instead of too dense from cream alone. 2% milk also works.
Sour Cream: Keeps the mash soft and velvety after chilling. Skipping it makes the mash firm and less smooth when reheated.
Salt: Salt the boiling water for the potatoes and season the onions as they caramelize. After everything is mashed and the onions are folded in, taste while the mash is still warm. This is the best time to adjust seasoning. Warm potatoes absorb salt evenly, once they cool, the flavor doesn’t distribute as well

Potatoes: Any type of potato works, the texture just varies slightly but the flavor stays the same

Yellow/Gold: Creamiest and best for make-ahead; stay silky after chilling.
Russet: Soft and fluffy when fresh; firm a little in the fridge but soften again when reheated with a bit of milk or cream.
Red: Slightly dense; add a little extra dairy when reheating.
White: Mild and soft; may dry slightly after chilling, so add more cream when warming.

Step by step photos: French onion mashed potatoes

These visual instructions help you see the process. You can also find the full printable recipe with ingredient amounts and instructions at the end of the post.

Sliced onions cooking in butter, slowly caramelizing in a pan until golden and soft.

In a wide pot, melt butter and add olive oil. Add sliced onions, salt, sugar, and thyme.
Cook for about 30-40 minutes, stirring often, until golden and soft. This is where all the flavor begins.

Optional (recommended): Scoop out 2–3 tablespoons of the caramelized onions and set them aside. You’ll use these for topping later when serving.

Creamy French onion mashed potatoes with caramelized onions and thyme

Potato chunks boiling in salted water, then fully cooked potatoes shown in the pot before draining.

Boil the peeled and cut potato pieces in salted water.
This photo shows the potatoes right after boiling — before draining. The edges should look soft and starting to break apart. Go ahead and drain them now.

Hot cooked potatoes with butter, sour cream, and half-and-half added on top, beginning to be mashed together.

Add the butter, heavy cream, whole milk and sour cream to the hot drained potatoes while they’re steaming, the warmth helps everything blend smoothly.
Mash gently for 2–3 minutes to break down the potatoes and work the dairy through. They’ll look uneven at first, that’s normal.

Mashed potatoes at the thin, very soft stage before thickening, then mash becoming creamy and stable as it’s worked.

The “don’t panic” stage : At this point the mash looks very soft and thinner than you expect. That's totally fine. This happens because the hot potatoes haven’t absorbed the dairy yet.
Keep mashing and stirring for another 1–2 minutes. As the potatoes cool slightly, the starch absorbs the dairy and the sour cream helps the mixture shift from thin → creamy → stable.

Caramelized onions being added to mashed potatoes and folded through until evenly combined.

Add the caramelized onions. Fold the remaining onions into the mash.
Gently fold for 2–3 minutes until the onions are worked through evenly.

If serving the same day, let the mash rest for 20–30 minutes — it settles and thickens. Spoon into a serving dish and top with the reserved onions before serving.

If making ahead: Spoon the mash into a dish, let it cool slightly, cover tightly, and refrigerate for up to 2 days. When ready to serve, reheat gently, then finish with the reserved onions on top.

Side-by-side comparison of mashed potatoes: on the left, freshly made with soft caramelized onion swirls; on the right, the chilled next-day version, firmer in texture before reheating.

Freshly Made texture (Same Day): this is how the mash looks about an hour after it’s made, smooth and silky, If you’re serving it right away, add the reserved onions on the top now.
Next day texture: Here’s the mash the next day, straight from the fridge — not even reheated yet. It stays soft and smooth because of the butter, dairy, and sour cream. Reheat, then finish with the reserved caramelized onions before serving.

Reheating instructions

You can reheat the mashed potatoes straight from the fridge.

Oven: Drizzle 2–3 tablespoons milk or cream over the top (don’t stir it in). This helps the mash heat evenly and keeps the surface from drying out. Cover with foil and warm at 350°F (175°C) for 30–40 minutes. Remove the foil for the last 10 minutes. Stir before serving and finish with the reserved onions.
Microwave: Place in a microwave-safe bowl, add 2–3 tablespoons milk or cream, heat on 50% power for 3–5 minutes, stir, then continue in short bursts until hot.
Stovetop: Warm over low heat, stirring often. Add a little milk or cream until smooth and creamy again.

Storage

Fridge: Store the mashed potatoes in an airtight container or covered dish in the fridge for up to 2 days.

The butter, sour cream, and dairy help the mash stay soft and smooth. When you’re ready to serve, reheat gently and add 2–3 tablespoons of milk or cream to bring back the silky texture. Reheat first, then add the reserved onions on top.

Freezing isn’t recommended for this recipe. Because the mash contains butter, cream, and sour cream, the texture becomes grainy once thawed. It keeps its best texture when refrigerated for up to 2 days.

Watch how to make the recipe

Recipe