In a small bowl, combine mustard, honey, and 2 teaspoons olive oil.
Heat 1 teaspoon olive oil in an oven safe sauté pan or dutch oven.
Add the onion and sauté over medium-high heat until softened, about 5 minutes. Add the garlic and sauté for an additional 1 - 2 minutes, until fragrant.
Place the chicken tenderloins on top of the onion and garlic mixture, or if the pan you cooked the onion and garlic in is not oven safe, transfer the onion and garlic to an oven safe dish and add the chicken to it. Salt and pepper the tops of the chicken according to your personal preference.
Pour the honey mustard mixture on top of the chicken. Arrange the rosemary sprigs around the chicken in the pan.
Bake the chicken covered at 400 degrees F for 20 minutes. Then remove the cover, baste the chicken with the sauce, and continue to cook for an additional 20 minutes uncovered, or until the chicken is browned on top and cooked through.