In a large resealable plastic bag, combine the olive oil, vinegar, mustard, dried herbs, and garlic or onion powder if using. Seal the bag tightly and shake well until the marinade ingredients blend into a smooth, fragrant mixture.

Open the bag and add the chicken breasts, ensuring they are evenly coated with the marinade. Massage the bag gently to distribute the flavors throughout. Reseal and let the chicken marinate in the refrigerator for at least 1 hour for best results, or freeze immediately for later use (up to 2 weeks).

When ready to cook, thaw frozen chicken overnight in the refrigerator, or use a cold-water bath for quicker defrosting. If short on time, microwave on 30 percent power for one-minute intervals until pliable but still cold.

Preheat a grill or grill pan over medium-high heat. Once hot, place the marinated chicken on the grill and cook for about 4 minutes per side, or until the internal temperature reaches 165°F (74°C) and the juices run clear. For an oven-baked version, arrange the chicken on a lightly oiled baking sheet and bake in a preheated 375°F (190°C) oven for approximately 15 minutes, or until fully cooked.

Let the chicken rest for a few minutes before slicing to preserve its juiciness. Serve warm with your favorite sides such as roasted vegetables, rice pilaf, or a crisp garden salad for a flavorful, balanced meal.