Prepare steak: Pound garlic, salt, and pepper into a paste, using a mortar and pestle. Add oil and smear all over steak. Cut steak into 5- to 6-in. pieces and set aside.
Make salsa: Combine ingredients and set aside.
Make bean spread: Toast cumin in a small frying pan over medium heat until fragrant, 1 to 2 minutes. With motor running, drop garlic into food processor to mince. Add beans, cumin, chile flakes, oil, and lemon juice; whirl until smooth (mixture will be thick). Set aside.
Heat a grill to high (450° to 550°). Grill steak, turning once, until done the way you like, 5 to 6 minutes for medium-rare. Transfer to a board. Crisp rolls on grill, turning often, about 1 minute; set on board.
Drain salsa and add 2 tbsp. liquid to beans. Split rolls and smear cut sides with bean spread. Slice steak across the grain, pile into rolls, and top with salsa and cilantro.
*Find at well-stocked grocery stores and Latino markets.