Cheesy Mustard Baked Chicken

boneless, skinless chicken breasts

butter, divided

Kosher salt and freshly ground black pepper

heaping tablespoons shallots, minced

garlic clove, pressed or minced

all-purpose flour

whole milk

whole grain mustard

extra sharp white cheddar cheese (4 ounces), grated

Parmesan cheese, grated and divided

panko bread crumbs

Italian flat leaf parsley, chopped, for garnish