1-First Step: Preheat the oven to 375 degrees F and prepare a baking dish with non-stick cooking spray. In a mixing bowl, whisk together the whole grain mustard, Dijon mustard, yellow mustard, honey, and 1 tablespoon of olive oil to create a smooth sauce.

2-Second Step: Heat the remaining 1 tablespoon of olive oil in a small skillet over medium heat. Add the chopped yellow onion and sauté for about 8 minutes until it turns golden brown, then stir in the minced garlic and cook for 30 seconds more. Spread this onion and garlic mixture evenly in the prepared baking dish for a flavorful base.

3-Third Step: Season the chicken breasts or thighs with salt and pepper on both sides, then place them over the onion layer. Pour the honey mustard sauce over the chicken, making sure each piece is well coated, and lay the rosemary sprigs between the pieces.

4-Fourth Step: Bake for 25 to 30 minutes, or until the chicken reaches an internal temperature of 165 degrees F. If the sauce starts to burn, cover with foil and spoon the sauce over the chicken before serving.

5-Fifth Step: Let it rest and serve.