Make the pickled vegetables: Put carrot and daikon in a small bowl and sprinkle with sugar and salt. Add rice vinegar, toss well and set aside.
Make the dipping sauce: In a small bowl, stir together brown sugar, vinegar, lime juice, fish sauce, and serrano pepper. Stir in 1/2 cup cold water and let mixture sit for 15 minutes. (Leftover sauce will keep up to 3 days, refrigerated.)
Marinate the shrimp: Put shrimp in a shallow dish. Add fish sauce, brown sugar, garlic and lemongrass. Mix well to coat. If you are going to grill, place the shrimp on skewers.
Meanwhile, bring a large pot of water to boil. Turn off heat and add rice noodles. Soak noodles, stirring occasionally, until softened, usually about 7 to 8 minutes. Drain and rinse with cold water. Leave in colander at room temperature.
Shred lettuce leaves and herb sprigs, place at the bottom of each bowl. Keep cool, covered with a damp towel.
Put oil in a wok or frying pan over high heat. When oil is hot, add shrimp without crowding (work in batches if necessary). Cook for about 2 minutes per side, until lightly browned.
To serve, divide the noodles among the 4 large soup bowls, then top each with hot shrimp, pickled vegetables and a tablespoon or so of dipping sauce. Sprinkle with scallions and peanuts (and beans sprouts if using). Serve with extra herbs, lime and Sriracha.
Course Main, Salad
Cuisine Asain, Bowl, Noodles, Salad
For Vegan Option: Use tofu instead of shrimp. Replace fish sauce with coconut aminos.