2 tablespoons unsalted butter
4 teaspoons all-purpose flour
1 1/4 cups whole milk
1/4 teaspoon salt
1/8 teaspoon ground black pepper
Pinch ground nutmeg
1 (15-ounce) container whole milk ricotta cheese
1 (10-ounce) package frozen chopped spinach, thawed, squeezed dry
1 cup plus 2 tablespoons grated Parmesan
3 ounces thinly sliced prosciutto, chopped
1 large egg, beaten to blend
3/4 teaspoon salt, plus more for salting water
1/2 teaspoon freshly ground black pepper
1 to 2 tablespoons olive oil
12 uncooked lasagna noodles
2 cups marinara sauce
1 cup shredded mozzarella (about 4 ounces)