Special equipment: butcher's twine; an instant-read thermometer
For the rice noodles: In a large bowl, combine the rice vinegar, lime juice, fish sauce, sesame oil, garlic, ginger and Fresno and serrano chiles and stir together. Toss in the carrots and cucumbers and let stand for 15 minutes for the flavors to meld.
Bring a large pot of water to the boil. Prepare the noodles according to package instructions. Drain and rinse the noodles with cold running water. Toss the noodles with the vegetables and dressing and refrigerate until serving.
For the flank steak: Prepare a grill for direct and indirect heat: For gas grills (with 3 or more burners), turn all the burners to medium-high heat; after about 15 minutes, turn off one of the side burners and turn the remaining burners down to medium. For charcoal grills, bank one chimney starter-full of lit and ashed-over charcoal briquettes to one side of the grill. Set up a drip pan on the other side to avoid flare-ups. (Be sure to consult the grill manufacturer's guide for best results.) If your grill has a thermometer, aim to keep the temperature at about 350 degrees F throughout grilling.
Set the flank steak on a cutting board. Starting at the thin, narrow end of the steak, slice it in half horizontally to within 2 inches of the opposite wide end. Open it up like a book so you have 1 long piece. Pound the steak with a mallet to an even thickness of about 1/2 inch. Cover the steak with the curry paste and season with 1 teaspoon salt. Sprinkle with the peanuts, leaving a 1-inch border along the edges. Arrange half of the cilantro and scallion greens in a line, followed by half of the tomatoes and then half of the scallion whites. Repeat with the remaining ingredients. Roll up the steak like a jelly roll, starting at the short end. Tie the roll closed around the circumference with butcher's twine at 2-inch intervals. Tie one more piece of twine lengthwise around the meat. Sprinkle the outside of the meat with salt and pepper.
Put the steak on direct heat and cook, turning, to brown all 4 sides, about 2 minutes per side. Move the steak to indirect heat, cover and cook, turning occasionally, until the very center reads 115 degrees F on an instant-read thermometer for a steak that is well done on the outside and medium rare in the center, 25 to 30 minutes more. (Test frequently; the time will vary based on the temperature of your grill.) Let the steak rest 5 to 6 minutes.
Untie the steak and cut it into thin slices. Garnish the noodles with chopped nuts and cilantro leaves. Serve with the lettuce leaves on the side.
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