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French Onion Potato Gratin Recipe

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4.3 from 4 reviews

French Onion Potato Gratin is a rich and comforting dish featuring thinly sliced russet potatoes layered with deeply caramelized French onions, infused with fragrant fresh thyme, and topped with a luscious blend of Gruyere and Parmesan cheeses. Baked covered and finished under the broiler for a bubbly golden crust, this gratin makes for an elegant side or hearty main to elevate any meal.

Ingredients

For the Caramelized French Onions

  • 2 tablespoons unsalted butter
  • 1 pound yellow onion, thinly sliced
  • ½ teaspoon kosher salt
  • ¼ teaspoon black pepper
  • 2 sprigs fresh thyme
  • 2 cloves garlic, minced
  • ½ cup beef broth
  • 1 teaspoon balsamic vinegar

For the Gratin

  • 3 pounds russet potatoes, peeled and cut into 1/8 inch slices
  • 1 ½ cups shredded Gruyere cheese (about 6 ounces)
  • ½ cup freshly grated Parmesan cheese (about 2 ounces)
  • 2 sprigs fresh thyme, leaves removed
  • ¾ cup heavy cream

Instructions

  1. Caramelize Onions: Melt the butter in a 12-inch heavy-bottomed skillet over medium heat. Add the thinly sliced onions, fresh thyme sprigs, kosher salt, and black pepper. Cook, stirring frequently, until the onions are very soft and deep golden-brown, about 25 to 30 minutes. Reduce heat and stir more often if onions begin to burn.
  2. Deglaze Pan: Add the minced garlic and cook for 1 minute until fragrant. Pour in the beef broth and scrape up any browned bits from the pan. Bring broth to a boil, then reduce heat and simmer until the liquid reduces by half. Remove from heat, discard thyme sprigs, and stir in the balsamic vinegar.
  3. Prepare Oven and Baking Dish: Preheat your oven to 400°F (200°C). Generously butter a 12-inch casserole dish or cast iron pan to prevent sticking.
  4. Mix Cheese and Thyme: In a small bowl, combine shredded Gruyere, grated Parmesan, and the removed fresh thyme leaves. Set aside two-thirds of this mixture for later use.
  5. Combine Potatoes, Onions, and Cream: In a large mixing bowl, toss the thin potato slices with the caramelized onions, heavy cream, and two-thirds of the cheese and thyme mixture. Gently break apart the potato slices to ensure each is well coated with the creamy onion mixture.
  6. Assemble the Gratin: Arrange the potato slices upright in tight sideways stacks inside the prepared baking dish, adding extra slices if needed to keep them stable. Spoon any leftover cheese and onion mixture over the stacks, then pour any remaining cream from the bowl evenly over the top.
  7. Bake Covered: Cover the dish tightly with foil and bake in the preheated oven for 60 minutes, allowing the potatoes to cook through and the flavors to meld.
  8. Broil and Rest: Remove the foil and sprinkle the remaining cheese mixture evenly over the gratin. Place under the oven broiler for 5 to 6 minutes until the cheese bubbles and develops a golden-brown crust. Remove from oven and let rest for 15 minutes to set before serving. Garnish with additional fresh thyme if desired.

Notes

  • Use uniform potato slices for even cooking, ideally 1/8 inch thick.
  • Caramelizing onions low and slow brings out their natural sweetness and depth of flavor.
  • Pre-soaking potatoes in cold water can help remove excess starch for a creamier texture, but is optional.
  • Adjust baking time depending on your oven and thickness of potatoes; bake until potatoes are tender when pierced with a knife.
  • For a vegetarian version, substitute beef broth with vegetable broth.
  • Leftovers can be refrigerated for up to 3 days and reheated gently.

Find it online: https://cookedbysarah.com/french-onion-potato-gratin-recipe/