Directions
Preheat your oven to 375°F (190°C). Lightly grease a 9×13-inch glass casserole dish with 1 tablespoon butter or cooking spray. Set aside.
Caramelize the onions: In a large skillet, melt 2 tablespoons butter over medium heat. Add the sliced onions and 1/2 teaspoon of the salt. Cook, stirring occasionally, for 20–25 minutes, lowering the heat if they start to brown too quickly. If they seem dry or start to scorch, add 1 tablespoon olive oil and a splash of water. Sprinkle in the optional 1 teaspoon sugar if you’d like to speed up caramelizing. Continue cooking until the onions are soft, golden brown, and sweet-smelling. Remove from heat and let cool slightly.
Make the creamy pour-over: In a medium bowl or large measuring cup with a spout, whisk together the heavy cream, 1 cup grated Parmesan, remaining 1 teaspoon salt, black pepper, and chopped thyme. Stir in the warm caramelized onions until everything is well combined. This is your one creamy pour-over mixture.
Prepare the potatoes: Peel the potatoes if you like (you can leave the skins on for more texture and nutrients). Slice them into thin rounds, about 1/8-inch thick, using a sharp knife or a mandoline. Try to keep the slices even so they cook at the same rate.
