In a large skillet over medium heat, add olive oil and sauté chopped onion and minced garlic until softened, about 5 minutes. Then add ground beef and season with salt, pepper, basil, and oregano. Cook until browned, about 8-10 minutes.

In your slow cooker, spread a thin layer of marinara sauce at the bottom, layer half of the frozen ravioli, half of the cooked beef mixture, and sprinkle with mozzarella and ricotta. Repeat layers, finishing with sauce and mozzarella.

Pour water around the edges of the slow cooker to create steam. Secure the lid and cook on low for 4-6 hours or high for 2-3 hours until ravioli is tender and cheese is melted.

In the final 10 minutes, sprinkle grated Parmesan cheese over the top. Cover and let it melt into a golden layer.

Carefully scoop portions into bowls or plates and serve hot, with garlic bread or a fresh salad.