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French Onion Mashed Potatoes

PREP TIME 20 minutes
COOK TIME 45 minutes
TOTAL TIME 1 hour 5 minutes
servings6 -8 servings
Rich, creamy, and full of flavor, with golden caramelized onions swirled into buttery mashed potatoes.
Top view of French onion mashed potatoes swirled with caramelized onions and garnished with fresh thyme.
Ingredients

French Onion Base

  • 2 Tablespoons (28 g) butter
  • 3 medium onions (500 g) sliced medium-thick
  • 1 Tablespoon (15 ml) olive oil (or canola or any neutral oil)
  • 1 teaspoon white or brown sugar
  • ½ teaspoon salt
  • 1 teaspoon dried thyme or 1 Tablespoon 15 ml fresh thyme (or Italian seasoning)

Mashed Potatoes

  • 3 pounds 1.3 kg potatoes, peeled and cut into 1-inch (2.5 cm) cubes (Yukon Gold or other yellow potatoes, best)
  • 1½ teaspoons salt
  • 4 Tablespoons (56 g) butter
  • ¼ cup (60 ml) whole milk
  • ¼ cup (60 ml) heavy cream (see note 2)
  • ½ cup (120 g) sour cream
  • ½ teaspoon black pepper
Instructions

Make the French onion base

  • Melt the butter in a wide pan over medium heat.
  • Add the sliced onions, oil, sugar, salt, and thyme.
  • Cook on medium-low heat for 30–40 minutes, stirring often, until golden, soft, and deeply caramelized. Slow cooking brings out the sweetness and full flavor.
  • Scoop out 2–3 Tablespoons of the onions and set aside for topping.

Cook the potatoes

  • Place the potatoes in a large pot, cover with water, and add the salt.
  • Bring to a boil and cook until soft and easily pierced.
  • Drain well, then let the steam escape for a minute so the potatoes stay light and fluffy.

Mash and mix

  • Add the butter, whole milk, heavy cream and sour cream to the hot drained potatoes.
  • Mash gently for 2–3 minutes. The mash will look very soft and thinner than expected at first. This is normal. Hot potatoes have not absorbed the dairy yet.
  • Mash for 1–2 minutes more until creamy and stable. Add the black pepper and fold in the caramelized onions.
  • Taste while warm and adjust seasoning if needed. Warm mash absorbs salt evenly.

If serving the same day

  • Let the mash rest for 20–30 minutes to settle and thicken. Spoon into a serving dish and top with the reserved caramelized onions.

Make ahead (up to 2 days)

  • Spoon the mash into a dish, let it cool slightly, cover tightly, and refrigerate for up to 2 days. Reheat, then finish with the reserved caramelized onions on top.

Reheat and serve

  • Oven (best): Drizzle 2–3 Tablespoons (30–45 ml) milk or cream over the top (don’t stir it in)
  • 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 add the reserved onions.
  • Microwave (small portions): Place in a microwave-safe bowl. Add 1 Tablespoon (15 ml) milk or cream on top (don’t stir). Heat on 50% power for 3–5 minutes, stir, then continue in short bursts until hot. Top with the reserved onions.
  • Stovetop: Place in a pot and add 1 Tablespoon (15 ml) milk or cream on top. Warm over low heat, stirring occasionally, until smooth and creamy again. Finish with the reserved onions.
Recipe Notes
  1. Heavy cream may be called double cream (UK) or thickened/whipping cream (Australia). Anything around 30–36% fat works the same.
 
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Nutrition
Serving: 1serving | Calories: 399kcal | Carbohydrates: 47g | Protein: 6g | Fat: 22g | Saturated Fat: 12g | Polyunsaturated Fat: 1g | Monounsaturated Fat: 7g | Trans Fat: 0.5g | Cholesterol: 54mg | Sodium: 894mg | Potassium: 1092mg | Fiber: 6g | Sugar: 6g | Calcium: 85mg | Iron: 2mg