Melt butter in skillet, add onion. Cook until see through. Add garlic.

Add brown sugar and mustard. Simmer until everything is melted together.

Cut each chicken breast in half to make them thinner.

Place chicken in baking dish and pour sugar mixture over it, making sure each breast is fully coated.

Bake 25 - 30 min @ 350° or until chicken is fully cooked. Be sure not to overcook or chicken will be dry.