01 - Heat olive oil in a large skillet over medium heat. Sauté chopped onion until translucent, approximately 3 minutes. Add minced garlic and cook 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. 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. Mix until evenly 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 - Add half the uncooked ziti pasta, followed by half the browned sausage, half the ricotta mixture, and approximately 2 cups of marinara sauce. Repeat layers with remaining pasta, sausage, ricotta, and sauce.
06 - Sprinkle the 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 let rest for 10 to 15 minutes to set. Garnish with fresh basil leaves before serving.