01 - Heat olive oil in a large skillet over medium heat. Sauté onion until translucent, approximately 3 minutes. Add minced garlic and cook for 1 additional minute until fragrant.
02 - Add sausage to skillet, breaking apart with a wooden spoon. Cook until thoroughly browned and no pink remains. Drain excess fat and set aside.
03 - Combine ricotta, half the mozzarella (1 cup), Parmesan, dried basil, oregano, and crushed red pepper flakes in a large bowl. Season generously with salt and pepper, mixing until fully incorporated.
04 - Coat slow cooker insert with non-stick spray. Spread 1 cup marinara sauce evenly across bottom. Layer half the uncooked ziti, half the browned sausage, half the ricotta mixture, and approximately 2 cups marinara sauce.
05 - Repeat layers with remaining pasta, sausage, ricotta mixture, and marinara sauce. Finish by topping with remaining mozzarella cheese.
06 - Cover and cook on LOW setting for 3 to 4 hours, until pasta is tender and cheese is melted and bubbly.
07 - Remove lid and let rest 10 to 15 minutes before serving to allow layers to set. Garnish with fresh basil if desired.