Preheat oven to 350℉.

To a large skillet over medium heat, add the butter and onion. Once simmering, put the lid on the pan and turn the heat to low. Let the onions cook for 10-12 minutes, stirring often, until golden brown. Remove onions with a slotted spoon and set aside, keeping the excess drippings in the pan.

Season the chicken on each side with paprika, salt, and pepper. Turn the heat to medium-high and add in the chicken to the pan you cooked the onions in. Sear for 2-3 minutes on each side to brown the chicken on the outside. This will create a golden brown crust on the outside of the chicken (the chicken does not need to be cooked through at this point).

Add the cooked onions, French onion soup, water, and balsamic vinegar and let simmer for 2-3 minutes to thicken the sauce.

Grease a medium-sized casserole dish with nonstick spray. Add the entire mixture to the baking dish and cover with mozzarella cheese. Bake for 25-30 minutes, or until the chicken is cooked through. Serve over mashed potatoes, rice, or egg noodles.