Bring 1-1/2 cups water to a boil in a small saucepan over high heat. Add the rice and salt, reduce the heat to low, cover, and simmer gently until the rice is tender and the water absorbed, about 20 minutes.
Meanwhile, arrange the lettuce leaves in a bowl and set the cilantro, mint, radishes, and scallions in separate piles on a plate. In a small bowl, combine 1/4 cup of the hoisin with 2 Tbs. water; reserve for assembly.
Position an oven rack 5 to 6 inches from the broiler element and heat the broiler to high. Line the bottom of a broiler pan with foil and replace the perforated top part of the pan. Season the steak with a large pinch of salt and the pepper. Broil the steak until it loses its raw appearance on top and begins to brown, about 3 minutes. Brush the steak with about 2 Tbs. of the remaining hoisin, and broil until lightly browned, 2 minutes longer. Turn the steak over with tongs and broil until it begins to brown, about 3 minutes. Brush generously with the remaining hoisin, and broil 2 to 3 minutes longer for medium rare (130°F on an instant-read thermometer), or to desired doneness (don’t let the hoisin burn; if necessary, move the pan to a lower rack).
Transfer the steak to a cutting board, sprinkle with the sesame seeds, and let rest for 5 minutes. Cut into thin slices across the grain at a slight angle. Arrange the steak and rice in separate bowls. Let diners assemble their own packages, filling the lettuce leaves with the rice, steak, cilantro, mint, radishes, scallions, and a drizzle of the diluted hoisin.