01 - Heat olive oil in a large skillet over medium heat. Sauté chopped onion until translucent, about 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 browned and no longer pink, approximately 5-7 minutes. Drain excess fat and set aside.
03 - In a large bowl, combine ricotta cheese, 1 cup mozzarella, Parmesan, dried basil, oregano, and crushed red pepper flakes. Season generously with salt and pepper. Mix 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: add half the uncooked ziti, followed by half the browned sausage, half the cheese mixture, and approximately 2 cups marinara sauce. Repeat with remaining pasta, sausage, cheese filling, and sauce.
06 - Sprinkle the remaining 1 cup of shredded mozzarella 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. Avoid overcooking to prevent mushy pasta.
08 - Remove lid and let the ziti rest for 10 to 15 minutes before serving—this allows the sauce to thicken slightly and makes serving easier. Garnish with fresh basil leaves if desired.