Cook the ziti pasta according to package instructions until al dente. Drain and set aside.
In a large skillet over medium heat, add the ground beef and chopped onion. Cook until the beef is browned and the onion is softened, about 6-8 minutes. Drain excess fat.
Add the minced garlic to the skillet and cook for 1 minute until fragrant. Remove from heat.
In a large bowl, combine the cooked ground beef mixture, marinara sauce, dried Italian seasoning, salt, and black pepper. Stir well to combine.
Lightly grease the inside of the crock pot.
Add half of the cooked ziti pasta to the crock pot. Spread half of the ricotta cheese evenly over the pasta layer.
Pour half of the meat sauce mixture over the ricotta layer.
Sprinkle 1 cup of shredded mozzarella cheese evenly over the sauce.
Repeat the layering with the remaining ziti pasta, ricotta cheese, meat sauce, and mozzarella cheese.
Sprinkle the grated Parmesan cheese evenly on top.
Pour 2 cups of water around the edges of the pasta in the crock pot to help keep the pasta moist during cooking.
Cover and cook on low for 4 to 4 1/2 hours, until the cheese is melted and bubbly and the pasta is tender.
Turn off the crock pot and let the baked ziti rest for 10 minutes before serving.