In a medium mixing bowl, combine the olive oil, balsamic vinegar, rosemary, Dijon mustard, salt, and pepper. Whisk vigorously until the mixture is emulsified and evenly blended.

Place the chicken in the bowl with the marinade. Toss gently to ensure the chicken is fully coated on all sides.

Cover the bowl and let the chicken marinate at room temperature for 30 minutes to 1 hour. If refrigerating, allow it to marinate for up to 5 hours. Avoid over-marinating to prevent a mushy texture.

When ready to cook, bring the chicken back to room temperature for about 30 minutes to ensure even cooking.

Cook the chicken using your preferred method. For grilling, set the temperature to medium and use oiled grates to prevent sticking. Ensure the internal temperature reaches 165°F (74°C) for food safety.