01 - Heat olive oil in a large skillet over medium heat. Sauté onion until translucent, about 3 minutes. Add garlic and cook for 1 minute more until fragrant.
02 - Add Italian sausage to the skillet, breaking apart with a wooden spoon. Cook until browned and no longer pink. Drain excess fat and set aside.
03 - In a large bowl, combine ricotta, 1 cup mozzarella, Parmesan, dried basil, oregano, and red pepper flakes. Season generously with salt and pepper.
04 - Coat slow cooker insert with non-stick spray. Spread 1 cup marinara sauce evenly across bottom. Layer half the uncooked ziti, half the browned sausage, half the cheese mixture, and 2 cups sauce.
05 - Repeat layers with remaining pasta, sausage, ricotta mixture, and marinara sauce. Top with remaining 1 cup mozzarella cheese.
06 - Cover and cook on LOW setting for 3 to 4 hours, until pasta is tender and cheese is melted and bubbly.
07 - Let ziti rest 10 to 15 minutes before serving to allow layers to set. Garnish with fresh basil if desired.