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 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, half the mozzarella (1 cup), grated Parmesan, dried basil, oregano, and crushed red pepper flakes. Season generously with salt and pepper. Stir until thoroughly 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 - Begin layering: spread half the uncooked ziti pasta, followed by half the browned sausage, half the ricotta mixture, and approximately 2 cups marinara sauce. Repeat layers with remaining pasta, sausage, ricotta, and sauce, ensuring even distribution.
06 - Sprinkle remaining mozzarella cheese evenly over the top layer.
07 - Cover and cook on LOW setting for 3 to 4 hours, until pasta reaches desired tenderness and cheese is fully melted and bubbly.
08 - Remove lid and allow baked ziti to rest for 10 to 15 minutes before serving. This allows layers to set for easier portioning. Garnish with fresh basil leaves if desired.