01 - Heat olive oil in a large skillet over medium heat. Sauté chopped onion until translucent, about 3 minutes. Add minced garlic and cook for 1 minute until fragrant.
02 - Add Italian sausage to the skillet, breaking it apart with a wooden spoon. Cook until browned and no longer pink, approximately 5-7 minutes. Drain excess fat and set aside.
03 - In a large bowl, combine ricotta cheese, 1 cup shredded mozzarella (half of total), grated Parmesan, dried basil, dried oregano, and crushed red pepper flakes if using. Season generously with salt and black 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 of the uncooked ziti pasta over the sauce base. Top with half of the browned sausage mixture, followed by half of the ricotta cheese mixture. Spread approximately 2 cups of marinara sauce over the cheese layer.
06 - Repeat layering with remaining uncooked ziti, remaining sausage mixture, remaining ricotta mixture, and remaining marinara sauce. Ensure all pasta is covered with sauce for proper cooking.
07 - Sprinkle remaining 1 cup of shredded mozzarella cheese evenly over the top. 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 the baked ziti rest for 10 to 15 minutes before serving. This allows the dish to set and makes serving easier. Garnish with fresh basil leaves if desired.