Sheet Pan Layerless Lasagna

Ingredients

• 4 tablespoons olive oil, divided
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• 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