Pasta and Lamb Casserole (Pastitsio)

2 medium onion s, finely chopped

2 tablespoons olive oil

2 large garlic cloves, finely chopped

2 pounds ground lamb

1 (28-ounces) can whole tomatoes in juice

1 (15-ounces) can whole tomatoes in juice

4 thyme sprigs

1 1/4 teaspoons ground cinnamon

1 1/4 teaspoons ground allspice

1/2 teaspoon grated nutmeg

1/4 teaspoon ground clove s For bechamel sauce:

7 tablespoons unsalted butter

6 tablespoons all-purpose flour

7 cups whole milk

3/4 teaspoon grated nutmeg

1 cup grated Parmigiano-Reggiano or kefalotyri cheese (2 ounces)

5 large egg yolks For pasta:

2 pounds ziti

1 3/4 cups coarse fresh bread crumbs (from 4 slices firm white sandwich bread)

1/4 cup grated Parmigiano-Reggiano or kefalotyri cheese