Wash hands with warm, soapy water for 20 seconds.
Preheat oven to 300℉.
Spray a 9x13-inch baking pan with nonstick spray.
Place onion in the bottom of the baking pan.
In a bowl, mix together beans, brown sugar, ketchup, mustard, water, and black pepper.
Spoon bean mixture over onions and cover dish with aluminum foil.
Bake at 300℉ for 1 hour.