Classic Potato & Caramelized Onion, Gruyère Gratin

1. Prepare the caramelized onions. In a large skillet or sauté pan, melt 2 tablespoons of butter and sauté the sliced onions over medium heat. Once the onions start sweating, turn the heat down to medium-low. Stir the onions occasionally, as needed. Allow them to cook slowly for 35-45 minutes until a deep brown color is formed and they become fully caramelized. Add the garlic and cook for another 2-3 minutes. Add the thyme leaves and stir allowing the fragrance to bloom.
2. While the onions cook, prep the potatoes. Use a sharp peeler to peel the potatoes. Rinse off, then slice into 1/8” thick slices using a handheld mandoline.
3. Preheat the oven to 400° F.
4. Prepare the roux. In a separate, smaller saucepan, heat the remaining 4 tablespoons of butter. Once the butter has melted, add the flour and whisk continuously until the mixture is completely smooth, and as it cooks for about 1 - 2 minutes. This process removes the raw flour flavor and helps create a silky sauce.
5. Create a Béchamel sauce by very slowly pouring milk into the roux. Continue whisking the mixture until the sauce is smooth. Add in the salt and pepper and continue whisking over medium-low heat for 3-5 minutes or until the sauce thickens and becomes very creamy.
6. Remove from heat and transform the sauce into a Mornay sauce by slowly adding in the shredded Gruyère cheese. Whisk until the sauce is smooth, and the cheese is well incorporated. The sauce will continue to thicken while maintaining its creamy texture.
7. Layer the gratin. Butter the bottom and sides of a large baking dish and lay a single layer of potatoes, then a layer of caramelized onions. Pour some of the sauce over the potatoes and continue to layer the potatoes, onions, and sauce to form additional layers. Repeat this step until all of the potatoes are used (or until the layers reach within 1/2” from the top of the dish).
8. Top the final layer of potatoes with the remaining onions, then sauce, and finally sprinkle with the reserved 1/4 cup of cheese.
9. Cover the gratin with a tight-fitting lid or foil and bake for 60 minutes. Remove the lid or foil, and continue to bake for another 10 minutes, or until the top turns golden brown and the gratin is hot and bubbling.
10. Garnish with additional fresh stems of thyme, and serve while hot.

Mornay Sauce Ingredients:

• 4 tablespoons butter
• 1/4 cup all-purpose flour
• 3 cups whole milk
• 1-1/2 teaspoons Kosher salt
• 1/4 teaspoon black pepper
• 2 cup grated Gruyère, 1/4 cup reserved for topping the gratin