In a resealable plastic bag, combine fish sauce, lime juice, brown sugar, garlic, red pepper flakes, and black pepper. Add steak, seal bag, and marinate in the refrigerator (turning occasionally) for 1 hour or up to 8 hours.

In a mixing bowl, whisk together water, vinegar, sugar and salt. Toss in carrots and radishes and let the mixture sit for at least 1 hour.

Heat grill or grill pan to medium. Grill steak to desired doneness, turning once, about 4 to 6 minutes per side for medium-rare. Let rest 5 minutes before slicing.

Spread mayonnaise on toasted baguettes, then top with sliced steak, pickled vegetables, cucumber slices and cilantro.