1. Bring 8 cups (2 L) water to a boil in a large pot set over high heat. Add 1 tablespoon (15 ml) fish sauce. Add the shrimp and cook until just firm, 2-3 minutes. Remove the cooked shrimp to a cutting board and cool for 3 minutes. When cool enough to handle, peel, devein, and cut in half lengthwise. Set aside.
2. Whisk together lime juice, remaining fish sauce, chili paste, and sugar to make a dressing. Dress the noodles and toss well to combine.
3. Fill a pie dish with warm water. Dip 1 rice paper wrapper into the water for 10-15 seconds, until the wrapper is pliable and slightly tacky, then transfer to a cutting board. Note that the hotter the water, the quicker the process, and that if too hot, there is greater risk of tearing and breaking.
4. Over the softened wrapper, lay 3-4 slices of the shrimp, pink side down, in a line across the middle, so they can be seen through the wrapper. Over the shrimp, add a pinch of the lettuce, then noodles, veggies, then herbs. Bring the bottom edge of the wrap tightly over the filling, and then fold in the 2 sides. Finish rolling from bottom to top until the entire wrapper is rolled, being careful not to tear the rice paper. Place on a plate and cover with a damp towel to prevent drying.
5. Repeat process with the remaining wrappers until the filling is gone, keep cool until ready to serve.
Combine garlic, chilies, cilantro, onions, sugar, cashews, oil, fish sauce, and lime juice in a blender, and blend until smooth.
For a nice presentation, dot a black slate serving dish with 5 small spoonfuls of the Nam Jim, cut a chilled spring roll into 5 pieces and present in a line over the sauce.