Preheat oven to 425ºF. Spray a large baking dish with nonstick cooking spray.
In a small mixing bowl, whisk together whole grain mustard, Dijon mustard, honey, olive oil, crushed rosemary, garlic powder, and ¼ teaspoon salt.
Sprinkle both sides of chicken with remaining ¼ teaspoon salt and pepper and place in prepared baking dish. Pour mustard mixture on top of chicken. Cover baking dish with foil.
Bake for 20 minutes, remove the foil, and spoon the sauce over the chicken.
Bake for another 22 minutes with the foil removed, or until internal temperature of the chicken reads 165ºF on an instant read thermometer when taken in the thickest part of the breast.
Serve immediately with sauce on top.
Store leftover chicken tightly covered for up to 3 days in the fridge or a month in the freezer.
To reheat, put chicken and sauce in a baking dish and heat for 15 minutes at 350°F or until heated through. You can also reheat in a skillet on the stove over medium-low heat. We don't recommend the microwave for reheating chicken as it tends to dry it out and make it tough.
Substitute boneless skinless chicken thighs. Or use bone in chicken thighs or breasts, with the skin removed. Chicken tenders are good, too. Since they take less time to cook, you can make them in a skillet on the stove. Adjust the amount of sauce if you're cooking a smaller amount of chicken.