*available at Asian markets Marinate the beef: In a blender blend together the garlic, the fish sauce, the sesame oil, the sugar, and the salt, in a shallow dish pour the marinade over the steak, turning the steak to coat it well, and let the steak marinate, covered and chilled, for 4 hours or overnight. Make the sauce: In the blender blend together the water, the garlic, the fish sauce, the lime juice, the sugar, and the red pepper flakes. In a large bowl soak the noodles in warm water to cover for 5 minutes and drain them. In a kettle of salted boiling water cook the noodles for 5 minutes and drain them in a colander. Rinse the noodles well under cold water and drain them well. Grill the steak, discarding the marinade, on an oiled rack set 4 to 5 inches over glowing coals or broil it under a preheated broiler about 3 to 4 inches from the heat for 8 to 10 minutes on each side, or until it is springy to the touch, for medium-rare meat. Transfer the steak to a cutting board, let it stand for 10 minutes, and holding a knife at a 45-degree angle slice it thin. Divide the cucumber, the lettuce, and the herbs among 8 large plates, mixing them, and mound the noodles over the mixture on each plate. Arrange the steak decoratively over the noodles and sprinkle it with the peanuts. Spoon some of the sauce over each plate and serve the remaining sauce separately. Serves 8. Gourmet September 1993