For the marinade, pound the garlic, sugar, and shallots in a mortar into a paste. Add the fish sauce, sesame seeds, black pepper, sesame oil, and lemon grass. (The lower third of the stalk of lemon grass should be sliced thinly, then minced, before adding to the mortar. It's best to mince in a food processor).
Slice beef into thin crosswise slices, 1/8-inch thick, place in a bowl and cover with marinade for about 1 hour.
Preheat a ridged grill or broiler and place the meat on the hot grill. Cook for 1 minute on each side. Cool to room temperature.
Cook the rice sticks in boiling water for about 4 minutes. Drain, rinse, and set aside.
To serve, set out 4 to 6 bowls and arrange the food in the following manner:
Place some shredded cucumber and lettuce in the bottom of each bowl, then sprinkle on some of the mint and coriander. Distribute the noodles among the bowls. Arrange the meat over the noodles. Sprinkle with more mint and cilantro, plus the chopped peanuts. Add a few pieces of shredded hot pepper on top and decorate with a sprig of cilantro.
Serve with a bowl of nuoc cham to sprinkle on top.
For Nuoc Cham, peel garlic. Split the chile and remove seeds and membrane. Cut into pieces and put into mortar with garlic and sugar. Pound to a paste. Squeeze the lime juice into the paste and with a small knife remove the pulp from the lime section and add it as well. Mash this mixture and add the fish sauce and water.