Chop the pork barbecue and reheat in a pan with the barbecue sauce until lightly caramelized.
Mix together the mayonnaise and grainy mustard.
Rinse the lettuce, dry in a salad spinner and tear or cut into small pieces.
Cut the baguette into three portions. Split each portion into halves. Pull out the soft centers to make room for the filling (this makes assembly so much easier and the sandwiches will be easier to handle too). Optionally, toast the bread.
Spread the cavities of both halves with the mayo-mustard mixture.
Lay the bottom halves of the baguette portions flat. Pile on the pork barbecue. Sprinkle fried shallots and torn basil on the pork.
Lay the top halves of the baguette, cut side up, on your work surface. Fill with lettuce.
Carefully invert the lettuce-filled top halves on the pork filled bottom halves. Serve immediately.