Preheat oven to 350F
Melt butter in a skillet and add sliced onions. Saute onions until they become translucent. Remove from pan and set aside.
Season chicken breasts with paprika, garlic powder, and salt. Turn heat to medium-high, add cooking spray to skillet, and brown chicken breasts on both sides, about 5 minutes on each side.
Add sauteed onions back to skillet
Open the can of soup and add it to the chicken and onions. Cook for 5 minutes or until soup thickens a little bit and the chicken is well covered in the sauce
If you are using a cast-iron skillet, divide the mozzarella cheese evenly over each chicken breast and put the pan in the oven.
If you are not using a cast iron pan, transfer the chicken, onions, and sauce to a baking dish.
Insert baking dish in the oven and bake for 15 to 20 minutes, or until cheese is melted and sauce is boiling.