Start by preheating your oven to 375°F. While the oven warms up, whisk together the coarse mustard, Dijon mustard, yellow mustard, honey, and one tablespoon of olive oil in a bowl. This combination creates a sweet and tangy sauce that’s the heart of the recipe.
Heat the remaining tablespoon of olive oil in a small skillet over medium heat. Add the chopped onion and sauté for about 7-8 minutes until they turn golden and soft. Toss in the garlic and sauté for another 30 seconds, just enough to release its aroma. Spread this mixture evenly across the bottom of your baking dish.
Season the chicken breasts with salt and freshly ground black pepper on both sides. Arrange them in a single layer on top of the onion and garlic mixture in your baking dish.
Evenly pour the mustard sauce over the chicken, using a basting brush if needed, to coat each piece thoroughly. Place the rosemary sprigs between the chicken breasts to infuse the dish with their earthy, savory fragrance. You can also topped it with cheese if you want.
Bake the chicken in the preheated oven for 25-30 minutes, checking halfway through to ensure the sauce isn’t browning too much. If needed, tent the dish with aluminum foil. You’ll know the chicken is perfectly done when its internal temperature reaches 165°F at the thickest point.
After removing the dish from the oven, allow the chicken to rest for about five minutes. This helps the juices redistribute, keeping the meat tender and juicy. Serve warm with a generous drizzle of the sauce.