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 additional minute until fragrant.
02 - Add Italian sausage to the skillet, breaking it 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, 1 cup shredded mozzarella, grated Parmesan, dried basil, oregano, and crushed red pepper flakes. Season generously with salt and pepper, mixing until well incorporated.
04 - Coat the slow cooker insert with non-stick cooking spray. Spread 1 cup of marinara sauce evenly across the bottom to prevent sticking.
05 - Arrange half the uncooked ziti over the sauce base. Top with half the browned sausage, followed by half the ricotta mixture. Drizzle approximately 2 cups of marinara sauce over the layer.
06 - Repeat layering with remaining uncooked pasta, sausage, ricotta mixture, and marinara sauce. Ensure all pasta is covered with sauce for proper cooking.
07 - Sprinkle the remaining 1 cup of shredded mozzarella cheese evenly over the top layer.
08 - Cover and cook on LOW setting for 3 to 4 hours, until pasta is tender and cheese is melted and bubbly. Avoid removing the lid frequently to maintain consistent heat.
09 - Turn off the slow cooker and let the ziti rest for 10 to 15 minutes before serving. This allows the dish to set and makes serving easier. Garnish with fresh basil if desired.