Place the uncooked chicken breast in a casserole dish and season generously with salt and pepper.

In a measuring cup or small mixing bowl, combine balsamic vinegar, mustard, and garlic. Whisk until combined.

Pour mixture over chicken and turn until fully coated. Marinate for at least 20 minutes or up to 1 hour. 

Wash and dice new potoatoes into bite-sized pieces (about 1 inch in diameter), and add toa medium-sized mixing bowl.

Add chopped onion and chopped rosemary to the same bowl. Drizzle with olive oil and season with salt and pepper. Stir to combine and set aside.

Preheat oven to 425 degrees fahrenheit.

Add olive oil to a 10-inch cast iron skillet over medium-high heat. When the skillet is hot sear the chicken on both sides - about 2 minutes per side.

Add the potatoes, onion, rosemary to the skillet of chicken and spread evenly around the skillet. Add the remaining sprigs of rosemary on top of chicken.

Transfer the skillet to the oven and bake until potatoes are tender and chicken is cooked through - about 20 minutes. (uncovered)

Serve the chicken and potatoes with pan drippings.