1. Heat olive oil in a skillet over medium heat. Add onion and garlic, sautéing until softened. Add ground beef, season with salt, pepper, and Italian seasoning, and cook until browned. Drain any excess fat and mix in the marinara sauce. Remove from heat.
2. In a separate bowl, mix ricotta cheese, egg, parsley, and a pinch of salt and pepper until well combined.
3. Spread a thin layer of the beef and sauce mixture at the bottom of the slow cooker.
4. Layer half of the frozen ravioli over the sauce, followed by half of the ricotta mixture, half of the mozzarella, and a sprinkle of Parmesan.
5. Repeat the layers, ending with a generous layer of mozzarella and Parmesan.
6. Cover and cook on low for 4-6 hours, until the ravioli is tender and the cheese is bubbling.
7. Garnish with additional chopped parsley before serving.
