Put 2 cups hot water in a large bowl and add the noodles. Soak them until softened but slightly resilient, 20 to 30 minutes. Drain them and pat dry on paper towels. Put in a bowl and cover.
Fill a large bowl with hot water. Place a clean kitchen towel next to the bowl. Submerge 1 rice paper wrapper in the water and soak it until softened, about 1 minute. Carefully remove the rice paper from the water and lay it flat on the towel.
In the center of the sheet, layer 4 lettuce pieces, about 20 sprouts, 4 shrimp halves, arranged side by side in a row, 4 mint leaves, 4 cilantro sprigs, and one-eighth of the carrots. Pull the bottom up over the filling, fold over the sides, and roll up like an egg roll. Place seam-side-down on a plate and repeat the process with the remaining rice papers and filling.
Serve immediately, with the Hot Mustard Sauce.
Yield: 8 summer rolls