Marinade: Combine all ingredients and mix with flank steak in a plastic bag.
Refrigerate for at least 2 hours.
Heat grill to medium-high heat.
Remove flank steak from marinade and place on grill.
Allow the steak to sear 5 minutes on first side and 3 to 5 minutes on other side for medium-rare (or until desired temperature).
Remove and allow to rest for 10 minutes.
Assemble:Lay out lavosh sheets on table and spread an even layer of coarse-ground mustard on each.
Thinly slice steak across the grain and place equal amounts on each of the lavosh to form an even layer.
Top with a layer of avocado, tomato, cilantro and romaine, and season with salt and pepper.
Roll up each sandwich in the form of a burrito and wrap in foil.
To reheat, place on a grill and serve with your favorite sauce.