Step 1: Brown the beef and onion - Heat a skillet over medium heat and add the ground beef and sliced onion. Cook until the beef is browned and the onion is softened. Break the beef up as it cooks so it stays nice and crumbly. Drain off any excess grease so the sauce does not become heavy.
Step 2: Transfer to the slow cooker - Add the cooked beef and onions to your slow cooker. This is where everything starts coming together, so make sure it is evenly spread out.
Step 3: Build the sauce - Sprinkle in the beefy onion soup mix, then pour in the beef broth and heavy cream. Stir everything together until well combined. At this point it already smells pretty incredible.
Step 4: Slow cook the base - Cover and cook on low for about 3 hours. This gives the flavors time to meld and the sauce to develop that deep savory taste.
Step 5: Add the ravioli - Gently stir in the frozen cheese ravioli, making sure they are submerged in the sauce. Be careful here so you do not tear them.
Step 6: Finish cooking - Cover again and cook for another 30 to 40 minutes, just until the ravioli are tender. Give it a gentle stir once if needed.
Step 7: Add the cheese and serve - Lay the swiss cheese slices over the top. Cover for a few minutes until the cheese melts. Sprinkle with parsley and crispy onions if using, then serve warm.