Place chicken breasts in bottom of slow cooker. Slice onion thinly, add on top of chicken. Add soup mix, can of condensed soup, and broth. Cover and cook on high for 3-4 hours, until chicken internal temperature reaches 165F.
Remove chicken from slow cooker and put in oven safe pan or cast iron. Top with onions, reserving liquid in the slow cooker. Top each breast with 2 slices of cheese and broil in the oven for 4-5 minutes until cheese is melted and begins to brown.
To serve, top with additional onions and a few spoonfuls of liquid from slow cooker. Serve with your choice of sides and enjoy!