Marinate the beef
In a blender, combine garlic, fish sauce, sesame oil, sugar, and salt. Pour into a shallow dish, then add the steak, turning to coat it well, and let the steak marinate, covered and chilled, for 4 hours or overnight.
Make the sauce
In the blender, combine the water, garlic, fish sauce, lime juice, sugar, and red pepper flakes.
Cook the noodles
In a large bowl, cover noodles in warm water and soak for 5 minutes. Drain them in a kettle of salted boiling water, cook noodles for 5 minutes, and drain them in a colander. Rinse noodles well under cold water and then drain well.
Remove steak from the marinade. Grill over 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.
Transfer steak to a cutting board, let stand for 10 minutes, and, holding a knife at a 45-degree angle, cut thin slices. Divide the cucumber, lettuce, and herbs and mix evenly among 8 large plates. Mound noodles over the mixture on each plate. Arrange steak decoratively over the noodles and sprinkle it with the crushed peanuts. Spoon some of the sauce over each plate and serve the remaining sauce separately.