Cook’s Note:

Assemble the gratin the day before you plan on serving it. This allows the starch from the potatoes to release into the cream for a thicker and creamier dish.

STEP 1

Combine the shredded cheeses and set aside. Pour the cream into a saucepan and add the garlic and thyme sprigs. Slowly warm over medium-low heat, until you see bubbles appearing around the edge of the pan and it is about to reach boiling point. Remove from the heat and transfer mixture to a 4-cup measuring jug or similar container. Stir in the nutmeg and leave to steep.

STEP 2

Stack ham slices and very loosely roll the stack lengthwise like a jelly roll. Slice the roll into ¼-inch slices and separate all the pieces. Add ½ tbsp of olive oil to a small sauté pan and fry the ham, stirring constantly over medium high heat until crispy. Remove to a plate lined with paper towels to absorb any excess oil.

STEP 3

Peel and slice the onions and potatoes about an 1/8 of an inch thick. (A mandolin or electric slicer produces the best results.) Submerge the potato slices into a bowl of cold, lightly salted water. Add the onions to the pan used for the ham and add 1 tbsp of olive oil. Sauté onions over medium heat until they just start to color.

STEP 4

With remaining olive oil or butter, grease the bottom and sides of a 13” paella pan or 9 x 12-inch baking dish. Arrange ¼ of the potato slices, slightly overlapping. Remove garlic and thyme sprigs from cream and pour ½ cup cream over potatoes. Use the back of a spoon to evenly distribute the cream. Sprinkle 1/3 of the sautéed onion, 1 cup shredded cheese, ¼ of the jamón Serrano, and season lightly with salt and pepper.

STEP 5

Repeat layering three more times, starting with the potatoes. Top final layer with remaining shredded cheese. Bake directly or cover pan with plastic wrap and refrigerate overnight.

STEP 6

Preheat oven to 375°F. If kept overnight, remove gratin from refrigerator and bring to room temperature. Remove plastic wrap and cover pan loosely with foil. Bake gratin 1 hour. Remove foil and bake until fork-tender, about 30 minutes more. Let potatoes rest 10 minutes before serving. Garnish with remaining jamón Serrano and fresh thyme leaves.

Serve warm as a tapa, side dish or with a crisp green salad for a lovely lunch.