Sausage Gravy Breakfast Lasagna

Name: Sausage Gravy Breakfast Lasagna
Ingredients: Kosher salt and freshly ground black pepper; 8 ounces lasagna noodles (about 12); 3 tablespoons olive oil, plus more for tossing and greasing the baking dish; 2 pounds bulk breakfast sausage, thawed; 1 bunch scallions, whites cut into 1/2-inch pieces and greens thinly sliced; 1/2 cup all-purpose flour; 4 cups whole milk; 1/2 teaspoon freshly grated nutmeg; 2 cups shredded low-fat mozzarella cheese; 8 ounces white Cheddar, shredded; 1/2 cup grated Parmesan; Two 10-ounce packages frozen chopped spinach, thawed and excess liquid squeezed out; 1/4 cup fresh parsley leaves, chopped