01 - Heat olive oil in a large skillet over medium heat. Sauté chopped onion until translucent, approximately 3 minutes. Add minced garlic and cook 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 cheese, half the mozzarella (1 cup), grated 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. Top with half the browned sausage mixture, followed by half the ricotta mixture, and approximately 2 cups of marinara sauce. Repeat layers with remaining pasta, sausage, ricotta, and sauce.
06 - Sprinkle the remaining 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 - Let the baked ziti rest for 10 to 15 minutes before serving to allow layers to set. Garnish with fresh basil leaves if desired.