Sprinkle steak spice on each side of flank steak. Grill on hot barbecue or indoor grill, 5-7 minutes per side. Remove and set aside.
Reduce grill heat to low. Toss vegetables in olive oil, balsamic vinegar and sea salt. Lightly grill vegetables until they just begin to char, watching closely so they don’t burn.
Carve flank steak, against the grain, into slices about ¼ inch (6 mm) thick.
For each sandwich, spread mayonnaise on inside of one slice of bread, and grainy mustard on inside of the other. Place half the steak and vegetables on the bread, cover with sliced cheese and second slice of bread. Butter the outside of both slices of bread and grill sandwich in a panini press until cheese is melted, about 4-5 minutes, longer if necessary. Alternatively, grill in a frying pan with a weighted pie plate on top (a can of tomatoes makes a good weight); flip after about 2 minutes. Cut in half. Makes 2-4 servings.