Combine apple juice, mustards, vinegar, oil, lemon juice and shallot in a medium bowl and mix well.
Place chicken in a single layer in a shallow non-reactive pan. Pour marinade over, cover and refrigerate 8 hours or overnight.
To cook, drain chicken, reserving marinade. Place chicken over medium-hot coals and grill 6-7 minutes per side, until internal temperature is 165 degrees Fahrenheit (74 C). While chicken is grilling, place reserved marinade in a saucepan over medium heat and bring to a boil (or use a fresh batch of marinade ingredients). Reduce heat and simmer 8 minutes, or until chicken has finished cooking. Spoon sauce over chicken and serve.