Step 1

Bring a medium potful of water to a rolling boil. Add the rice vermicelli and, stirring often, cook them until the strands are soft and white, but still resilient, 3 to 5 minutes. Don’t be tempted to undercook them, as they must be fully cooked to absorb the flavors of the dish. Rinse them in a colander under cold water just until they’re cool and the water runs clear. Let the noodles drain in the colander for 30 minutes, and then set them aside for up to 2 hours, unrefrigerated.

Step 2

In a large skillet, heat the oil over high heat. Add the shallots and garlic, quickly stirring until fragrant, about 30 seconds. Add the shrimp and lemongrass; stir-fry for about 2 minutes. Add the mushrooms and sprinkle with the fish sauce, sugar, and a little salt. Stir-fry until the shrimp is cooked and the mushrooms are wilted, another 2 to 3 minutes. Remove from the heat.

Step 3

Divide the lettuce, bean sprouts, cucumber, mint, and basil among four large soup or pasta bowls. Fluff the noodles with your fingers and divide them among the prepared salad bowls. Put the shrimp topping on the noodles and garnish each bowl with the peanuts and cilantro. Pass the nuoc cham at the table; each diner should drizzle about 3 Tbs. over the salad and then toss the salad in the bowl a few times with two forks or chopsticks before eating.