To make the Mustard Barbecue Sauce, mix all the ingredients in a pot. Bring to a low boil; then reduce the heat and simmer for 15 minutes, stirring often. Set aside.
Cut the sausage in half lengthwise, grill until cooked, and set aside.
Slice a whole loaf of french bread in half lengthwise. Pile the cooked sausage on the bottom half of the bread, and top it with the mustard barbecue sauce and slices of pepper jack cheese. Close the loaf with the top half of the bread.
Grill the entire loaf to crisp the bread and melt the cheese. You may either place the loaf directly on the grill or wrap it in aluminum foil first.
Remove the loaf from the grill and slice it into smaller sandwiches.