01 - Heat olive oil in a large skillet over medium heat. Sauté onion until translucent, about 3 minutes. Add garlic and cook for 1 minute more until fragrant.
02 - Add Italian sausage to the skillet, breaking it apart with a wooden spoon. Cook until browned and no longer pink throughout. Drain excess fat and set aside.
03 - In a large bowl, combine ricotta, half of the mozzarella (1 cup), Parmesan, dried basil, oregano, and crushed red pepper flakes. Season generously with salt and pepper, mixing until fully incorporated.
04 - Spray the slow cooker insert with non-stick cooking spray. Spread 1 cup of marinara sauce evenly across the bottom to prevent sticking.
05 - Layer half the uncooked ziti over the sauce, followed by half of the browned sausage mixture, half of the ricotta cheese mixture, and approximately 2 cups of marinara sauce.
06 - Repeat layering with remaining uncooked ziti, sausage, ricotta mixture, and marinara sauce, ensuring all pasta is covered with sauce.
07 - Sprinkle remaining mozzarella cheese over the top. Cover and cook on LOW for 3-4 hours until pasta is tender and cheese is melted and bubbly.
08 - Remove lid and let rest 10-15 minutes before serving to allow layers to set. Garnish with fresh basil leaves if desired.