Preheat oven to 425 degrees F. Grease 9 x 13 pan.
In small bowl, whisk together mustard, maple syrup, brown sugar and red wine vinegar.
Arrange chicken in baking dish, leaving space between pieces.
Liberally salt and pepper each chicken breast.
Pour sauce mixture over chicken, making sure to coat all chicken pieces with some of the sauce.
Bake 15 minutes, then open oven and with a ladle, gather some of the sauce from the pan and pour on chicken again.
Reduce heat to 400 degrees and cook 15 minute more until sauce is bubbly and golden brown and chicken registers 165 degrees F internal temperature.
Remove from oven and sprinkle with fresh Rosemary, if desired.
Cook's Note: This can be prepared in a crock pot. Cooking time is 3-4 hours on low.