2 tablespoons unsalted butter
1 large VIDALIA onion, THINLY sliced
2 to 3 cloves garlic, minced
3 1/4 pounds medium Yukon Gold potatoes (6 to 7 potatoes), cut into 1/8-inch rounds
FRESH ground sea salt and black pepper
1 1/2 cups grated Fontina cheese – about 8 ounces
1 cup grated Parmesan – about 5 ounces
2 cups heavy cream
2 tablespoons finely chopped fresh rosemary
1 tablespoon Dijon mustard
1 teaspoon lemon zest
- Preheat the oven to 450 degrees F.
- Spray a LARGE 18X13, rimmed baking sheet with non-stick cooking spray.
- Melt the butter in a large sauté pan over medium heat.
- Add the onions and cook, stirring occasionally, until lightly caramelized, 15 to 20 minutes; if the onions are browning too quickly, reduce the heat to low. When the onions are almost done, stir in the garlic and cook an additional 2 minutes to lightly toast. Remove from the heat.
- Layer half of the potatoes, shingle style in 6 or 7 even rows on the prepared baking sheet, making sure the sheet is completely covered.
- Season the potatoes with salt and pepper, and sprinkle with half of the fontina and Parmesan.
- Spread the caramelized onions evenly over the top.
- Shingle the remaining half of the potatoes over the onions, and season with more salt and pepper.
- Turn the heat to medium under the sauté pan.
- Add the cream, rosemary, Dijon and lemon zest, stir together and bring to a SLOW simmer.
- Season with salt and then pour the cream over the potatoes; tilt the baking sheet slightly in all directions to ensure even coverage. Top with the remaining Fontina and Parmesan.
- Cover the baking sheet with foil and bake for 15 minutes.
- Remove the foil and continue baking until the top is golden brown, the edges are crispy and the potatoes are tender, another 20 to 25 minutes.
- Let the cool for 15 minutes before serving.