In a large skillet over medium high heat, melt the butter and sauté the onions just until golden.

Remove them from the skillet and put them in the slow cooker.

Put the chicken breasts in the same skillet and brown them on both sides, just for a few minutes.

Remove them from the skillet and add them to the slow cooker.

Add in the beef broth, white wine, garlic, salt, and black pepper and stir.

Cover the slow cooker and cook on low heat for 4 hours.

Remove the chicken to a plate and set aside. Transfer the broth and onions in the slow cooker to a large skillet on the stove.

Raise the heat to high and with a wire whisk, stir in 2 tablespoons of flour to thicken the gravy.

Add the chicken to the skillet and put 2 thin slices of Swiss cheese on each chicken breast.

Cover the skillet for a minute or two until the cheese has melted. Garnish with chopped parsley.