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 pink remains. Drain excess fat and set aside.
03 - In a large mixing bowl, combine ricotta cheese, half the mozzarella (1 cup), Parmesan, dried basil, oregano, and 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 - Begin layering: half the uncooked ziti, followed by half the browned sausage, half the ricotta mixture, and approximately 2 cups marinara sauce. Repeat layers with remaining pasta, sausage, ricotta filling, and sauce.
06 - Sprinkle 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 - Remove lid and allow dish to rest for 10 to 15 minutes before serving. This sets the layers for easier serving. Garnish with fresh basil leaves if desired.