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 crumbled 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 shredded mozzarella (1 cup), grated Parmesan, dried basil, oregano, and crushed red pepper flakes. Season generously with salt and black 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 - Arrange half the uncooked ziti pasta over the sauce base. Top with half the browned sausage mixture, followed by half the ricotta cheese mixture. Pour approximately 2 cups of marinara sauce over the layers.
06 - Repeat layers with remaining uncooked pasta, sausage, ricotta mixture, and marinara sauce. Ensure all pasta pieces are covered with sauce for proper cooking.
07 - Sprinkle the remaining 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 fully melted and bubbly.
09 - Remove lid and let the baked ziti rest for 10 to 15 minutes before serving to allow layers to set. Garnish with fresh basil leaves if desired.