Pickle some carrot and daikon. (The receipt in the Momofuku book is good, but basically skinny matchsticks, brine with some rice vinegar, water, salt, sugar.)
Toast a crab'slength of split baguette.
Toss 1 crab/person* w/ some flour, salt, and pepper in a paper bag.
Fry crabs in pan w/ butter.
Spread split baguette w/ sriracha and mayo.
Put crab on one side, carrot and daikon on other, garnish w/ cilantro.
*You could double up, but one of the upsides of this receipt is that it makes a decent, if light, meal, out of one crab/person. A banh mi usually has more than one filling, but you do get some contrast of flavors and textures within a single crab.