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 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 remains. Drain excess fat and set aside.
03 - In a large mixing bowl, combine ricotta cheese, 1 cup of the shredded mozzarella, grated Parmesan, dried basil, dried oregano, and crushed red pepper flakes. Season generously with salt and black pepper, mixing until fully 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 - Arrange half of the uncooked ziti pasta over the sauce base. Distribute half of the browned sausage mixture evenly over the pasta. Spread half of the ricotta cheese mixture across the sausage layer. Pour approximately 2 cups of marinara sauce over the ricotta.
06 - Repeat the layering process with remaining ingredients: remaining uncooked ziti, remaining sausage, remaining ricotta mixture, and the rest of the marinara sauce. Ensure ingredients are distributed evenly.
07 - Sprinkle the remaining 1 cup of shredded mozzarella cheese over the top layer, covering the surface completely for a golden, bubbly finish.
08 - Cover and cook on LOW setting for 3 to 4 hours. Pasta is done when tender to the bite and cheese is melted and bubbly around the edges.
09 - Remove the slow cooker insert and allow the baked ziti 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.