Extra-Cheesy White Lasagna

Name: Extra-Cheesy White Lasagna
Prep time: 35 min
Cook time: 1 h
Total time: 1 h 35 min
Cuisine: Italian
Ingredients: 1 quart whole milk (4 cups), at room temperature; 15 dried lasagna noodles (not no-boil, about 2/3 of a 1-pound box); 1 pound baby spinach, or 1 (16-ounce) package frozen spinach; 1 medium yellow onion, diced; 3 cloves garlic, minced; 12 ounces low-moisture whole-milk or part-skim mozzarella cheese, shredded (about 3 cups); 2 ounces Parmesan cheese, grated (about 1 firmly packed cup grated on a Microplane or 2/3 cup store-bought); 1 tablespoon olive oil; 1 pound uncooked sweet or hot Italian sausage, casings removed if using links; 2 3/4 teaspoons kosher salt, divided; 1/2 teaspoon freshly ground black pepper, divided; 1 stick (8 tablespoons) unsalted butter; 1/2 cup all-purpose flour; Pinch ground nutmeg; 1 (15- to 16-ounce) container whole-milk ricotta cheese (about 2 cups)