01 - Heat olive oil in a large skillet over medium heat. Cook 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 pink remains. Drain excess fat and set aside.
03 - In a large mixing bowl, combine ricotta, half the mozzarella (1 cup), Parmesan, dried basil, oregano, and red pepper flakes. Season generously with salt and pepper, mixing until fully incorporated.
04 - Coat 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 over the sauce base. Top with half the browned sausage, followed by half the ricotta mixture. Add approximately 2 cups marinara sauce, spreading to cover ingredients.
06 - Repeat layers with remaining uncooked pasta, sausage, ricotta mixture, and marinara sauce. Finish by sprinkling the remaining mozzarella cheese evenly over the top.
07 - 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 allow dish to rest for 10 to 15 minutes before serving. This helps the pasta set and makes serving easier. Garnish with fresh basil leaves if desired.