In a skillet over medium heat, brown the ground beef or Italian sausage. Drain the excess fat, and add the diced onions and minced garlic. Sauté until fragrant and onions are translucent, about 3-4 minutes.

In a large bowl, combine the cooked meat mixture with the uncooked pasta, bell peppers, pizza sauce, diced tomatoes, Italian seasoning, salt, and pepper. Mix well to ensure everything is evenly combined.

Lightly spray the inside of your crock pot with cooking spray. Pour half of the pasta mixture into the crock pot, followed by half of the mozzarella cheese. Repeat the layering process with the remaining pasta and cheese.

If desired, scatter pepperoni slices and black olives on top of the last layer of cheese.

Cover the crock pot and cook on low for 4-6 hours, or on high for 2-3 hours. The casserole is done when the pasta is tender and the cheese is melted and bubbly.

Once cooked, let it sit for about 10 minutes before serving. Garnish with fresh basil or parsley, if desired.