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French Onion Mashed Potato Gratin

Creamy mashed potatoes topped with caramelized onions, fresh cracked black pepper, and baked until a gooey layer of provolone, gruyere, and parmesan cheese.

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Cuisine: American
Prep Method: Baked


Side Dish

Prep Time

10 Mins

Cook Time

35 Mins


Serves 8


Creamy mashed potatoes topped with caramelized onions, fresh cracked black pepper, and baked until a gooey layer of provolone, gruyere, and parmesan cheese.



Caramelized Onions

  • 6 cups (900g) Yellow Onions, peeled, sliced ¼ inch
  • 2 tablespoons (28.4g) Unsalted Butter
  • 2 teaspoons (2g) Salt
  • ½ tablespoon (3g) Freshly Ground Black Pepper
  • 1 quart (960ml) Vegetable Stock or Vegetable Broth
  • 2 tablespoons (30ml) Worcestershire Sauce (optional)
  • ½ teaspoon (~1g) Dried Thyme (Fresh Thyme can be subbed)

Buttermilk Whipped Potatoes

  • 1-1/3 cup (320ml) Water
  • 1 teaspoon (~1g) Salt
  • ¼ cup (56.5g) Unsalted Butter, softened
  • ¾ cup (180ml) Whole Milk (Any milk can be subbed), cold
  • 2-2/3 cup (135g) Instant Mashed Potatoes (Dehydrated Potato Flakes)
  • 1/3 cup (80ml) Buttermilk
  • ¼ cup (60g) Sour Cream


  • 8 ounces (226g) Provolone Cheese Slices
  • 4 ounces (113g) Gruyere Cheese, grated
  • 1/3 cup (30g) Parmesan Cheese, grated


  • As Needed Flakey Sea Salt (such as Maldon)
  • 1 teaspoon Fresh Thyme Leaves


  1. Preheat the oven to 375 F (190 C).
  2. Peel and slice the onions about ¼ inch thick.
  3. In a heavy bottomed pot over medium high heat, melt the butter and add the onions. Cook the onions for 2-3 minutes or until they are slightly translucent. Add the salt and pepper and continue to cook the onions until they begin to caramelize. At this stage its import to stir the onions regularly so they do not burn. Once the onions take on an even, dark brown color (about the color of milk chocolate and should take about 20 minutes), add the vegetable stock, Worcestershire sauce, and thyme, scraping the bottom of the pan to release any caramelized flavor. Reduce the heat and allow the sauce to simmer for about 5 minutes. Remove from heat.
  4. To prepare the mashed potatoes, heat the water to a boil and add the salt and the butter. When the butter is melted, add the cold milk and stir in the instant mashed potato flakes. Allow the mixture to sit for about a minute, then fluff with a fork. Gently fold in the buttermilk and the sour cream. Transfer the mashed potatoes into an oven proof serving dish.
  5. In the same oven proof dish, ladle the onions and sauce on top of the mashed potato layer.
  6. Arrange the cheese in the following order: take half the provolone slices and arrange them on the top of the onions in an even layer, follow with the shredded gruyere, grated parmesan, and another layer of the provolone (off set the provolone so the second layer fills any gaps from the first). Spray a piece of aluminum foil with nonstick spray. Place the foil (oiled side down) oven the casserole and place in the preheated 375 F (190 C) oven for 25 minutes. Remove the aluminum foil and bake for an additional 10-12 minutes or until the cheese is bubbly and the top is golden brown.
  7. Remove the casserole from the oven and garnish with the fresh thyme and flakey salt.
  8. Enjoy right away, its ok to burn your mouth, it’s part of the fun and a crucial part of the experience.

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