01 - Heat olive oil in a large skillet over medium heat. Sauté chopped onion until translucent, approximately 3 minutes. Add minced garlic and continue cooking for 1 additional minute until fragrant.
02 - Add Italian sausage to the skillet, breaking apart with a wooden spoon. Cook until thoroughly browned and no longer pink throughout. Drain excess fat and set aside.
03 - In a large mixing bowl, combine ricotta, 1 cup mozzarella, Parmesan, dried basil, oregano, and crushed red pepper flakes. Season generously with salt and pepper. Mix until fully 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 - Layer half the uncooked ziti pasta evenly in the slow cooker. Top with half the browned sausage, followed by half the ricotta mixture. Pour approximately 2 cups marinara sauce over the layer.
06 - Repeat layering with remaining ziti, sausage, ricotta mixture, and marinara sauce. Ensure pasta is evenly coated. Sprinkle remaining 1 cup mozzarella cheese 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 ziti to rest for 10 to 15 minutes before serving. This sets the texture and makes serving easier. Garnish with fresh basil leaves if desired.