For the flank steak:
Season the steak on both sides with salt and pepper. In a separate bowl add the marinade ingredients, whisking to combine Place steak in bowl of marinade, turning over to coat the meat. Cover with plastic and refrigerate for 1 hour or overnight
Remove steak from fridge 20 minutes before cooking. Heat an oiled cast iron skillet to medium high and once hot, add the steak, searing about 5 minutes on each side or until meat reaches 150F. Remove from pan and let rest for 15 minutes before slicing across the grain of the meat.
Toss veggies in salt and 1 tsp of sugar, massaging with your hands for about 5 minutes. We do this in a strainer so that the excess juices can drain out. After 5 minutes, rinse with cold water
Pack in the veggies tightly in a jar. In a liquid measuring cup, combine cold water, sugar, and vinegar Pour over veggies and let sit in the fridge for 30 minutes or for as long as a week!
Assembling the Banh Mi Sandwich
Slice baguette lengthwise and butter the inside. Place the buttered side up in an oven set to broil. Watch carefully and remove from oven as soon as golden brown. Spread 1 Tbsp. of mayonnaise on the bottom, layer cucumbers, jalapenos, ¼ c. pickled veggies, sliced steak. Drizzle approximately ½ tsp soy sauce over beef. Top with cilantro. Close sandwich, cut in half, and enjoy!