Combine mustard, soy sauce, honey and vinegar. Set sauce aside.
In a 9 x 13 inch pan, combine garlic and oil. Place chicken in pan skin side down.
If using thighs, bake covered at 400 degrees F (205 degrees C) for 25 minutes. If using breasts, bake covered at 400 degrees F (205 degrees C) for 10 minutes. Uncover, and turn chicken pieces over. Sprinkle with sesame seeds. Bake until no longer pink in center, about 15 to 20 minutes. Sprinkle grapes over chicken, and bake 5 minutes longer. Remove from oven, and arrange chicken and grapes on platter. Pass sauce when serving.