In an oven safe skillet, melt butter over medium heat.
Add in sliced onions and cook down until they are brown and soft, 15-20 minutes. Add salt.
Preheat oven to 400 degrees.
Remove onions to a side bowl, and add in chicken. Cook until brown, about 5 minutes on each side.
Take cooked chicken breasts out, and put onions back in. Add beef stock, and allow to cook off some of the liquid and for the flavors to mingle. Cook for about 5 minutes.
Add chicken back in, nestling it among the onions. Top chicken with cheese, and place in preheated oven.
Cook for 10 minutes, or until the cheese is brown and bubbly.