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 it apart with a wooden spoon. Cook until browned and no longer pink throughout. Drain excess fat and set aside.
03 - In a large bowl, combine ricotta, 1 cup mozzarella (half of total), Parmesan, dried basil, oregano, and crushed red pepper flakes. Season generously with salt and pepper, mixing until well incorporated.
04 - Spray the slow cooker insert thoroughly with non-stick cooking spray. Spread 1 cup of marinara sauce evenly across the bottom to prevent sticking.
05 - Layer half the uncooked ziti pasta over the sauce base. Top with half of the browned sausage mixture, followed by half of the ricotta cheese mixture. Spread approximately 2 cups of marinara sauce over the cheese layer.
06 - Repeat layering with remaining uncooked ziti, sausage, ricotta mixture, and marinara sauce, ensuring even distribution.
07 - Sprinkle the remaining 1 cup of shredded mozzarella cheese evenly over the top layer.
08 - Cover and cook on LOW setting for 3 to 4 hours, until pasta is tender and cheese is melted and bubbly.
09 - Remove lid and let rest for 10 to 15 minutes before serving to allow the dish to set. Garnish with fresh basil leaves if desired.