01 - Heat olive oil in a large skillet over medium heat. Sauté chopped onion until translucent, approximately 3 minutes. Add minced garlic and continue cooking for 1 additional minute until fragrant.
02 - Add Italian sausage to the skillet, breaking it apart with a wooden spoon. Cook until thoroughly browned and no longer pink. Drain excess fat before proceeding.
03 - In a large mixing bowl, combine ricotta, 1 cup of the mozzarella (half), Parmesan, dried basil, oregano, and crushed red pepper flakes if using. Season generously with salt and pepper and mix until well incorporated.
04 - Coat 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 ingredients in this order: half of the uncooked ziti, followed by half of the browned sausage, half of the ricotta mixture, and approximately 2 cups of marinara sauce. Repeat the layers using remaining pasta, sausage, ricotta filling, and sauce.
06 - Sprinkle the remaining 1 cup of shredded mozzarella cheese evenly over the top layer.
07 - Cover and cook on LOW setting for 3 to 4 hours, until pasta is tender and cheese is melted and bubbly.
08 - Remove the lid and allow the dish to rest for 10 to 15 minutes before serving. This helps the layers set and makes serving easier. Garnish with fresh basil leaves if desired.