In a large skillet over medium-high heat, melt butter and saute onions until golden brown.
Remove from skillet and add to slow cooker.
In the same skillet, brown chicken breasts on both sides over medium-high heat, 3-4 minutes.
Remove from skillet and add to slow cooker with onions.
Add beef broth, cooking sherry or wine, minced garlic, and salt.
Cover slow cooker and cook on low heat 4 hours.
Top with sliced swiss cheese and spoon onions & broth over the top to melt.
Serve over polenta or mashed potatoes.