Creamy Caramelized Onion Potato Bake

Preheat oven to 375°F (190°C) and grease a 9×13-inch glass baking dish. Melt 2 tbsp butter in a large skillet over medium heat. Add 2 thinly sliced onions and ½ tsp salt. Cook for 20 to 25 minutes, stirring occasionally and adding 1 tbsp olive oil or a splash of water if dry, until soft, deep golden-brown, and sweet. Remove from heat and cool slightly.

In a medium bowl or spouted measuring cup, whisk together 2 cups heavy cream, 1 cup grated Parmesan, 1 tsp salt, 1 tsp black pepper, and 2 tsp chopped thyme. Stir in the warm caramelized onions until uniformly distributed.

Slice 3 lbs potatoes into uniform ⅛-inch rounds using a sharp knife or mandoline. Arrange the sliced potatoes in even layers in the greased glass casserole dish so the liquid can circulate freely between slices.

Slowly pour the cream and caramelized onion mixture all over the potatoes. Gently nudge the slices with a spoon so the liquid seeps between layers. Sprinkle the remaining 2 tbsp Parmesan evenly over the surface.

Cover the glass dish tightly with aluminum foil. Bake on the middle oven rack for 45 to 55 minutes until a paring knife glides into the center potatoes with zero resistance.

Remove foil and continue baking uncovered for 15 to 20 minutes until bubbling, lightly golden, and crisp around the dish edges.

Let the casserole rest on a wire rack for at least 10 minutes to allow the bubbling cream matrix to set and thicken before spooning onto plates.