Savory Potato and Onion Gratin

1.Preheat your oven to 375°F (190°C).

2.Grease a 9x13 inch baking dish with butter to prevent sticking.

3.In a large skillet over medium heat, melt 1 tablespoon of butter and add the sliced onions. Cook, stirring occasionally, until the onions are caramelized and golden brown, about 15-20 minutes. Add the minced garlic and cook for an additional 1-2 minutes until fragrant. Remove from heat and set aside.

4.In a mixing bowl, combine the heavy cream, whole milk, chopped thyme, salt, and black pepper. Whisk until well combined.

5.Layer half of the sliced potatoes in the bottom of the prepared baking dish. Sprinkle half of the caramelized onions over the potatoes, followed by half of the Gruyère cheese and a third of the Parmesan cheese.

6.Pour half of the cream mixture over the first layer. Repeat the layering process with the remaining potatoes, onions, Gruyère cheese, and cream mixture. Top with the remaining Parmesan cheese.

7.Cover the baking dish with aluminum foil and bake in the preheated oven for 45 minutes. Remove the foil and bake for an additional 20-25 minutes, or until the top is golden brown and the potatoes are tender when pierced with a fork.

8.Let the gratin cool for about 10 minutes before serving. This will help the layers set and make it easier to cut into portions.