2 tablespoons extra virgin olive oil (EVOO)
8 thin slices prosciutto, halved crosswise
2 leeks—trimmed, halved lengthwise and cut crosswise 1/2 inch thick
Salt and pepper
Two 10-12-ounce packages fresh potato gnocchi
2 tablespoons butter
2 tablespoons flour
2 cups milk
Ground nutmeg, to taste
1 cup grated Fontina or Fontina Valle D'Aosta cheese (3 generous handfuls)
1/2 cup grated Asiago cheese
1/2 cup grated Parmigiano Reggiano cheese
In a large skillet, heat the EVOO, 2 turns of the pan. Working in batches if necessary, add the prosciutto in a single layer and cook, turning once, until crisp, about 2 minutes. Drain on paper towels. Add the leeks to the skillet and cook until tender, 8-10 minutes; season with salt and pepper.
Bring a large pot of water to a boil, salt it, add the gnocchi and cook until the gnocchi float to the surface, about 3 minutes.
Meanwhile, in a medium saucepan, melt the butter over medium heat. Whisk in the flour for 1 minute, then whisk in the milk; season with salt, pepper and nutmeg. Cook until thickened, 5 minutes. Stir in the Fontina and Asiago until melted.
Pre-heat the broiler. Drain the gnocchi and place in a casserole dish. Stir in the leeks and cheese sauce and top with the Parmigiano Reggiano. Broil the casserole until browned, 3 to 4 minutes. Top with the prosciutto.