Step 1: Brown the beef and soften the onions - In a skillet over medium heat, brown the ground beef with the thinly sliced onion. Cook until the beef is no longer pink and the onion has softened. Drain off any excess grease so the sauce does not get heavy later.
Step 2: Transfer the mixture to the slow cooker - Move the cooked beef and onion mixture into your slow cooker, spreading it out evenly so everything cooks nicely and the flavors can blend together.
Step 3: Stir in the broth, soup mix, and cream - Add the beefy onion soup mix, beef broth, and heavy cream to the slow cooker. Stir well so the seasoning dissolves and the sauce starts to come together smoothly.
Step 4: Slow cook until rich and fragrant - Cover and cook on low for 3 hours. This is when the onions really mellow out and the sauce starts to smell like classic French onion comfort.
Step 5: Add the ravioli gently - Drop the frozen cheese ravioli into the slow cooker and gently stir so each piece gets coated in the sauce without breaking apart.
Step 6: Cook until the ravioli are tender - Cover and cook for another 30 to 40 minutes, until the ravioli are soft and cooked through. Give them one gentle stir halfway through if needed.
Step 7: Melt the cheese on top before serving - Lay the Swiss cheese slices over the top, cover again, and let the cheese melt into a cozy layer before finishing with parsley and crispy onions if you like.