Bring a large pot of generously salted water to a boil. Cook pasta 2 minutes less than the package directions. Drain the pasta and reserve 1 cup of the starchy water.

Heat oil in a large oven-proof skillet. Add the ground sausage and cook for 5-6 minutes. Add the cherry tomatoes, salt, garlic, oregano, and red pepper flakes. Continue to cook for 3 to 4 minutes, or until the tomatoes start to burst and release some of their juices. Add the marinara sauce, spinach, ½ cup of reserved starchy water, ½ cup parmesan cheese, and fresh parsley. Bring the sauce to a simmer. Stir in the cooked pasta, along with the remaining ½ cup of starchy water. Top with the shredded mozzarella and bake for 8 minutes, or until the cheese is melted.