Lasagna Rolls

2 tablespoons unsalted butter

4 teaspoons all-purpose flour

1 1/4 cups whole milk

1/4 teaspoon salt

teaspoon ground black pepper

Pinch ground nutmeg

Lasagna :

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)