Sheet Pan Layerless Lasagna

Name: Sheet Pan Layerless Lasagna
Prep time: 10 min
Cook time: 50 min
Total time: 1 h
Cuisine: European, Italian, Mediterranean
Ingredients: 4 tablespoons olive oil, divided; 1/2 teaspoon kosher salt, plus more for the pasta water; 1 pound taccole pasta or lasagne pasta broken into 2-inch pieces; 1 clove garlic, chopped; 1/2 teaspoon red pepper flakes; 1 (5- ounce) container baby spinach; 1 cup part skim ricotta cheese; 1 pound spicy Italian sausage, casings removed; 1 red onion, diced; 4 cups jarred marinara sauce; 2 1/2 cups shredded low moisture mozzarella, divided; 1 1/2 cups freshly grated parmesan cheese, divided