Go Back

Scalloped Potatoes with Caramelized Onions

Print Recipe Pin Recipe
Prep Time 25 minutes
Cook Time 1 hour
Total Time 1 hour 25 minutes
Course Side Dish
Cuisine American, French
Servings 8

Equipment

  • Mandoline slicer or sharp knife For evenly slicing potatoes. A mandoline gives you precise, uniform slices which is key for even cooking.
  • Large skillet To caramelize the onions slowly and evenly.
  • Medium saucepan For making the roux and creamy cheese sauce.
  • 9x13-inch baking dish The perfect size to layer everything beautifully.
  • Whisk and wooden spoon For the sauce and stirring the onions.

Ingredients
  

  • 4 large russet potatoes peeled and thinly sliced (about 1/8-inch thick)
  • 2 large yellow onions thinly sliced
  • 3 tablespoons unsalted butter divided
  • 2 cloves garlic minced
  • 2 tablespoons all-purpose flour
  • 2 cups whole milk warm
  • 1 cup heavy cream
  • 1 ½ cups shredded sharp cheddar cheese
  • ½ cup grated Parmesan cheese
  • 1 teaspoon fresh thyme leaves or ½ tsp dried thyme
  • Salt and black pepper to taste
  • Olive oil for sautéing

Instructions
 

Caramelize the Onions

  1. In a large skillet, heat a tablespoon of olive oil and 1 tablespoon of butter over medium-low heat. Add the sliced onions with a pinch of salt and cook slowly, stirring occasionally, until deeply golden brown—about 25–30 minutes. Set aside once they’re sweet and jammy.
    sliced yellow onions slowly caramelizing

Make the Cheese Sauce

  1. In a saucepan, melt the remaining butter over medium heat. Add the minced garlic and cook for 1 minute. Stir in the flour to make a roux and cook for another minute. Slowly whisk in warm milk and cream until smooth. Simmer until slightly thickened, then stir in cheddar, Parmesan, thyme, salt, and pepper. Turn off heat once it’s creamy and cheesy.
    creamy cheese sauce being whisked

Assemble the Dish

  1. Preheat oven to 375°F. Grease your baking dish. Layer half of the sliced potatoes, then top with half of the caramelized onions and half of the cheese sauce. Repeat with remaining potatoes, onions, and sauce.

Bake

  1. Cover the dish with foil and bake for 40 minutes. Remove foil and bake an additional 20 minutes until the top is golden, bubbly, and you can easily pierce the potatoes with a fork.

Rest and Serve

  1. Let it sit for 10–15 minutes after baking to firm up. Sprinkle a little extra Parmesan or thyme on top if you’re feeling fancy.