3 heads of garlic, cloves separated but not peeled
1 tablespoon extra-virgin olive oil
1 quart half-and-half
1 tablespoon chopped thyme
1 teaspoon chopped rosemary
Salt and freshly ground pepper
5 pounds Yukon Gold potatoes, peeled and very thinly sliced
9 ounces aged Manchego cheese, coarsely shredded (2 cups)
5 ounces San Simón or smoked Gouda cheese, shredded (1 cup)
Preheat the oven to 375°. In a 9-inch cake pan, drizzle the garlic with the oil. Cover with foil and roast for 40 minutes, until tender. Let cool, then squeeze out the cloves.
Mash the garlic to a paste and transfer to a saucepan. Add the half-and-half, thyme and rosemary and bring to a boil. Simmer over very low heat until reduced to 3 cups, 20 minutes; season with salt and pepper.
Arrange one-fourth of the potatoes in the bottom of a 9-by-13-inch baking dish. Top with one-fourth of the shredded cheeses and drizzle lightly with the garlic cream. Repeat the layering with the remaining potatoes, cheese and cream. Pour any remaining cream on top and press the top layer of potatoes to submerge it.
Bake the gratin for about 1 1/2 hours, until golden and bubbling. Let cool for 20 minutes before cutting into squares and serving.
The gratin can be prepared through Step 3; refrigerate overnight. Return to room temperature before baking.