Preheat the oven to 375 F. Combine the whole grain mustard, Dijon mustard, yellow mustard, honey, and one tablespoon of olive oil in a small bowl. Make sure the honey is fully incorporated.
Heat 1 ½ tablespoons of olive oil in a small skillet and saute the chopped onion until golden brown. Add the garlic, red pepper flakes, and dried parsley and cook for 30 seconds more.
Spray a 12x7-inch baking dish with cooking spray and sprinkle the onion/garlic mixture evenly on the bottom.
Season the chicken breasts with salt and freshly grated pepper on both sides and place them into the baking dish over the onion mixture.
Pour the honey mustard sauce evenly over the chicken breasts, place a few sprigs of fresh rosemary in between them, and bake for 30 minutes, basting once after 15 mins.