Trim the darker green end and the base from the lemon grass and remove any withered parts. Finely chop the lemon grass. Place the beef in a single layer in the freezer for 15-20 minutes or until the outside is frozen. This will make it easier to slice. Slice the beef into 1/8-inch (0.3 cm) slices. Add the lemon grass to the beef along with the pepper, garlic, fish sauce and sugar. Mix well to coat the slices of beef. Cover and refrigerate for at least 1 hour and up to 24 hours.
Submerge the rice noodles in cold water for an hour until softened. Bring a large pot of water to a rapid boil. Drain the noodles and add to the boiling water. The noodles will become tender quickly, within 1-2 minutes. They should be slightly soft with a bit of a bounce. Drain and cool under cold water. Drain well. Do not toss with oil to prevent them from sticking together.
Make the nuoc cham: combine all of the ingredients together, stirring to dissolve the sugar. Cover and refrigerate.
Before serving time, place equal amounts of the lettuce, then the noodles in 4 large bowls. Prepare and toss the basil, mint, cilantro and green onion together.
Heat a large, heavy frying pan or wok over high heat until smoking. If you don’t have a fast and hot burner, cook the beef in two batches. Add the beef and onions. Stir fry until the beef is almost cooked through. Add the bean sprouts and toss until the sprouts are softened. Top each noodle bowl with the beef mixture, the cucumber and add a fluffy pile of the herbs. Sprinkle with the peanuts and serve with the nuoc cham on the side. Each diner seasons their bowl with the nuoc cham, tosses it all together, and adjusts to taste with the extra seasonings.