Our Favorite Lasagna With Sausage, Spinach, and Three Cheeses

Name: Our Favorite Lasagna With Sausage, Spinach, and Three Cheeses
Cuisine: Italian
Ingredients: 1 tablespoon extra-virgin olive oil; 1 pound sweet Italian sausage, casings removed; 1 medium onion, chopped; 3/4 teaspoon kosher salt; 1/4 teaspoon freshly ground black pepper; 4 garlic cloves, minced; 1/2 teaspoon red-pepper flakes, plus more to taste; 2 tablespoons tomato paste; Two 28-ounce cans whole tomatoes; 2 teaspoons dried oregano; Two 15-ounce containers part-skim ricotta cheese; 1 1/2 cups packed basil leaves; 1/2 cup grated Parmesan (about 2 ounces); 2 large egg yolks; 2 10-ounce packages frozen chopped spinach, thawed, or 1 1/2 pounds fresh spinach, steamed; Pinch kosher salt; Vegetable-oil cooking spray; 9 no-boil lasagna noodles (such as Barilla); 1 pound fresh mozzarella, grated (about 4 cups); 9 x 13 x 2-inch baking dish, preferably glass