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Cheesy Gruyère Scalloped Onions

Cheesy Gruyère scalloped onions bake sliced yellow onions in a creamy thyme sauce beneath a golden panko topping. This bubbling casserole makes a rich, savory side dish with tender onions and plenty of Gruyère.
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Prep Time 20 minutes
Cook Time 56 minutes
Resting 10 minutes
Total Time 1 hour 26 minutes
Servings: 8 servings
Course: Side Dish
Cuisine: American
Calories: 520
Ingredients Equipment Method Notes

Ingredients
  

For the onion mixture
  • 4 tbsp butter unsalted, divided
  • 3 lb yellow onions peeled and sliced 1/4 inch thick
  • 1 tsp salt kosher
  • 3 tbsp flour all-purpose
  • 2 cup milk whole
  • 1 cup cream heavy
  • 1 tsp thyme leaves fresh
  • 0.5 tsp black pepper
  • 0.125 tsp nutmeg ground
  • 8 oz Gruyère cheese shredded and divided
For the topping
  • 0.5 cup panko breadcrumbs

Equipment

  • 1 9x13-inch baking dish
  • 1 12-inch skillet

Method
 

Cook the onions and sauce
  1. Preheat the oven to 375°F.
  2. Grease a 9x13-inch baking dish with 1 teaspoon of the butter.
  3. Melt 2 tablespoons plus 2 teaspoons of the butter in a 12-inch skillet over medium heat, then add the onions and salt. Cover and cook for 10-12 minutes, stirring once or twice, until the onions begin to soften.
  4. Uncover and cook for 10-15 minutes, stirring often, until the onions are very tender and excess liquid has evaporated.
  5. Sprinkle the flour over the onions and stir for 1 minute, then gradually stir in the milk and heavy cream. Add the thyme, black pepper, and nutmeg, and simmer for 3-5 minutes, stirring, until the sauce thickens enough to coat the onions.
  6. Remove the skillet from the heat and stir in 6 ounces of the Gruyère, then transfer the mixture to the prepared baking dish.
Top and bake
  1. Melt the remaining 1 tablespoon butter and toss it with the panko. Sprinkle the remaining 2 ounces Gruyère over the onions, then scatter the panko evenly on top.
  2. Bake for 25-30 minutes, until the casserole is bubbling and the topping is golden brown.
  3. Let the casserole rest for 10 minutes before serving.

Notes

For evenly tender onions, slice them uniformly. Refrigerate leftovers for 3-4 days; freezing is possible, though the creamy sauce may change texture. For a gluten-free option, use gluten-free all-purpose flour and gluten-free panko.