Place chicken in a large baking dish. Spread mustard evenly over the chicken and sprinkle with diced onion. Coat lightly with half of the brown sugar and pour in enough orange juice to cover chicken. Add butter on top.
Bake for 45 minutes then remove leftover sauce from baking dish and pour into a saucepan. Sprinkle chicken with remaining 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 serve separately.
Instead of using chicken breast, i tried legs. I leave the chicken in the mixture for a few hours so the orange, onion & mustard can marinate together. Put the butter on & in the oven. When it comes out, its nice & juicy. Such a quick & easy recipe!! - 14 Jun 2009