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 bowl, combine ricotta cheese, half the mozzarella (1 cup), grated Parmesan, dried basil, oregano, and crushed red pepper flakes. Season generously with salt and pepper. Mix until well 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 - Arrange half the uncooked ziti pasta over the sauce base. Top with half the browned sausage mixture, followed by half the cheese mixture. Pour approximately 2 cups of marinara sauce over the layer.
06 - Repeat layering with remaining pasta, sausage, cheese mixture, and marinara sauce. Finish by sprinkling the reserved mozzarella cheese evenly over the top.
07 - Cover and cook on LOW setting for 3 to 4 hours. Pasta should be tender and cheese melted and bubbly. Avoid overcooking to prevent mushy texture.
08 - Remove lid and allow the dish to rest for 10 to 15 minutes before serving. This helps the layers set and makes serving easier. Garnish with fresh basil leaves if desired.