For the Noodles: Bring a large pot of water to a rolling boil over high heat. Drop in the noodles, then remove the pot from the heat and let stand for 10 minutes; use tongs or a slotted spoon and a fork to separate the noodles in the water so they cook evenly. When the noodles are tender, drain, rinse in cold water, then drain again. You’ll have about 6 cups of cooked noodles. Let stand while you prepare the remaining ingredients.
While the noodles are cooling, make the sauce: Combine the garlic, sugar and chili-garlic sauce in the bowl of a mortar and pestle; mash to a paste. Scrape the paste into a medium bowl, then stir in the fish sauce, water and lime juice, so the sugar dissolves. Use right away, or cover and refrigerate for up to 1 week.
To assemble: Set out four big Asian-style noodle bowls or pasta plates or soup bowls. Divide the ingredients evenly among the bowls: lettuce first, topped with 1 1/2 cups of noodles in each bowl. Top the noodles in each portion with chicken on one side and the cucumber, fresh herbs and any optional ingredients you’re using on the other side.
Sprinkle the scallions and peanuts over the chicken. Serve immediately.