Combine brown sugar, soy sauce, sherry wine (or apple juice), olive oil, ground ginger, ground mustard, and garlic powder in a bowl and mix well to create the teriyaki marinade.

Pour 1 cup of the marinade into a large ziplock plastic bag.

Add the turkey breast to the bag, seal, and turn to coat the turkey with the marinade.

Refrigerate the turkey in the marinade overnight, ensuring it is well-coated.

Cover and refrigerate the remaining marinade separately for basting.

The next day, drain and discard the marinade from the turkey breast.

Place the turkey breast on a rack in a shallow roasting pan.

Bake uncovered at 350 degrees Fahrenheit (175 degrees Celsius) for 2 to 2 1/2 hours, or until a meat thermometer reads 170 degrees Fahrenheit (77 degrees Celsius).

Baste the turkey breast every 30 minutes with the reserved marinade during the baking process.

Once cooked, let the turkey breast stand for 10 minutes before slicing and serving.