In a small bowl, combine the mustards, honey, and 2 teaspoons of the olive oil.
In a sauté pan or cast iron skillet, heat the remaining 2 teaspoon of olive oil over medium heat. Once hot, add the onion and sauté until tender, about 5 minutes. Add the garlic and sauté for an additional 30-60 seconds, until fragrant.
Place the chicken on top of the onion and garlic mixture, and season with salt and pepper. Pour the honey mustard mixture on top of the chicken. Arrange the rosemary sprigs around the chicken, cover with foil, and bake for 20 minutes. Baste the chicken with the honey mustard and cook, uncovered, for an additional 10-15 minutes or until slightly browned and chicken is cooked through.