Spray the inside crock of your slow cooker with non-stick spray. Pour the frozen ravioli into the slow cooker.
In a large bowl, whisk together the crushed tomatoes, tomato sauce, onion, garlic, Italian seasonings, and salt and pepper until combined. Pour sauce over the ravioli, gently stir to coat all of the ravioli with sauce.
Cover and cook on LOW heat for 4-5 hours (until ravioli is tender).
About 10-15 minutes before you're ready to eat, sprinkle the ravioli with shredded mozzarella cheese. Place the cover back on slow cooker and allow the cheese to melt