Rinse chicken and pat dry. Combine mustard, garlic, marjoram and maple syrup in a small bowl. Spread about 1 1/2 tablespoons mustard mixture evenly on top of each chicken thigh, being careful to cover as much of the surface as possible to form a "crust." Arrange chicken in a glass baking dish. Bake for 45 to 50 minutes, or until mustard mixture has formed a crust and is slightly hardened, and juices run clear when the chicken is pierced.