Place chicken in a 9x13 inch baking dish. Spread mustard evenly over the chicken and sprinkle with chopped onion. Coat lightly with 1/4 cup of the brown sugar and pour in enough orange juice to cover chicken. Add butter on top.
Bake in preheated oven for 45 minutes, then remove leftover sauce from baking dish and pour into a saucepan. Sprinkle chicken with remaining 1/4 cup brown sugar and return to oven.
Whisk flour into sauce in saucepan. Add any leftover orange juice and heat on high until the sauce thickens. Remove chicken from oven and place on a serving dish; pour sauce over the chicken or into a gravy boat, and serve.
