Place a large nonstick skillet over medium-high heat. Add ground beef and onion. Break meat apart as is cooks. Cook until no longer pink and then add garlic, Italian seasoning, crushed red pepper flakes, salt, pasta sauce and tomato sauce. Remove from heat.
In a medium bowl, stir together ricotta cheese, egg, basil, Parmesan cheese, and 1 cup of mozzarella cheese.
Pour 1/3 of sauce into the bottom of a greased 6-quart slow cooker. Scatter 1/3 of the uncooked ziti on top.
Spoon 1/2 of the ricotta cheese mixture over the ziti.
Repeat another layer of sauce, ziti, and the remaining ricotta mixture.
Repeat with remaining ziti and then the remaining sauce.Cover and cook on HIGH for 2 1/2 hours.
Remove lid and sprinkle the remaining 2 cups of cheese on top. Cover with the lid just long enough to melt the cheese.Serve.