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. Drain excess fat and set aside.
03 - In a large bowl, combine ricotta, 1 cup mozzarella, Parmesan, dried basil, oregano, and crushed red pepper flakes. Season generously with salt and 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 over the sauce. Distribute half of the browned sausage mixture, followed by half of the ricotta mixture. Pour approximately 2 cups of marinara sauce over the layer.
06 - Repeat layering with remaining pasta, sausage, ricotta mixture, and sauce. Top evenly with the reserved 1 cup of mozzarella cheese.
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 sets the texture for easier portioning. Garnish with fresh basil if desired.