Baked Honey Mustard Chicken

lbs - 2 1/4 lbs boneless skinless chicken breast halves ((4 - 6 medium chicken breasts))

whole grain (, coarse mustard)

dijon mustard

yellow mustard

+ 2 Tbsp honey

olive oil (, divided)

Salt and freshly ground black pepper

chopped yellow onion

garlic (, minced)

small sprigs rosemary