Preheat oven to 350 degrees.
In a large oven-proof skillet (you can use a regular skillet, but you will need to transfer to a baking dish before placing in the oven) melt butter over medium heat and add the sliced onions. Sauté the onions until softened and translucent, about 4 minutes. Set aside.
Clean the skillet and spray with non stick cooking spray.
Season each chicken breast with paprika, garlic powder, and seasoned salt. Place seasoned chicken breasts into prepared skillet.
Cook for 5-7 minutes on medium high, on both sides or until chicken breasts are golden brown.
Add onions back into the skillet with the chicken breasts.
Pour in the can of French onion soup. Simmer the chicken and onions in the soup for 2-3 minutes-this will allow the soup to caramelize the chicken.
If using an oven proof skillet, top each chicken breast with mozzarella cheese (otherwise transfer chicken, onions, and sauce into a 9x13 inch baking dish and top with mozzarella cheese.)
Place skillet or baking dish in preheated oven and bake for 10-15 minutes or until bubbling.