In a small bowl, whisk together the whole grain mustard, Dijon mustard, yellow mustard, honey, and 1 tablespoon of olive oil until well combined. Lightly coat a 13x9-inch baking dish with non-stick cooking spray and set aside.

Heat the remaining 1 tablespoon of olive oil in a small skillet over medium heat. Add the chopped onion and sauté for about 8 minutes, or until softened and golden. Stir in the minced garlic and cook for an additional 30 seconds. Transfer the onion and garlic mixture to the prepared baking dish, spreading it out in an even layer.

Season both sides of each chicken breast with salt and freshly ground black pepper. Arrange the chicken directly on top of the onion layer in the baking dish.

Evenly pour the mustard-honey mixture over the chicken, ensuring that each piece is thoroughly coated. Place the rosemary sprigs between the chicken breasts.

Bake in a preheated oven at 375°F (190°C) for 25 to 30 minutes, or until the internal temperature of the thickest part of the chicken reaches 165°F (74°C). If the sauce begins to darken too much during baking, loosely cover the dish with foil to prevent burning. Once done, spoon some of the sauce from the baking dish over the chicken before serving.