Ingredients:
- 3 lbs Yukon Gold potatoes, peeled and sliced to 1/8-inch thickness
- 1 large yellow onion, very thinly sliced
- 2 tbsp unsalted butter, for greasing the dish
- 2 cups heavy cream
- 1 cup whole milk
- 3 cloves garlic, smashed and peeled
- 4 sprigs fresh thyme
- 1 tsp kosher salt
- 1/2 tsp freshly cracked black pepper
- 1/4 tsp ground nutmeg
- 1.5 cups Gruyère cheese, freshly grated
- 0.5 cup Parmesan cheese, finely grated
Instructions:
- Preheat your oven to 350°F (180°C). Generously grease your baking dish with the 2 tbsp of unsalted butter, making sure to hit the corners.
- In a medium saucepan, combine the 2 cups heavy cream, 1 cup whole milk, 3 smashed garlic cloves, 4 sprigs of thyme, 1 tsp salt, 1/2 tsp pepper, and 1/4 tsp nutmeg.
- Heat the cream mixture over medium low until tiny bubbles form around the edges. Do not let it reach a rolling boil. Turn off the heat and let it sit for 10 minutes. Note: This allows the thyme and garlic to perfume the liquid.
- Peel the 3 lbs of Yukon Golds and slice them into 1/8 inch rounds. Keep them in a bowl covered with a damp paper towel so they don't turn brown.
- Arrange about one third of the potato slices in the bottom of the dish, slightly overlapping them like shingles. Scatter half of the sliced onion over them.
- Sprinkle about 1/2 cup of the grated Gruyère over the onions. This creates a cheesy glue between the layers.
- Add another third of the potatoes, the rest of the onions, and another 1/2 cup of Gruyère. Finish with the final layer of potatoes.
- Remove the garlic cloves and thyme sprigs from the cream. Carefully pour the warm cream mixture over the potatoes. Stop when the liquid reaches just below the top layer.
- Mix the remaining 1/2 cup of Gruyère with the 0.5 cup of Parmesan and sprinkle it evenly over the top.
- Place in the oven for 45 minutes until the sauce is bubbling and the top is deeply golden. Test for doneness by sliding a knife into the center; it should meet no resistance.