MAKE THE DOUGH (IF DOING SO): combine all ingredients, adding boiling water as needed until you get a sticky but workable dough. roll out into a thin sheet, cutting into 3 inch rounds.
MAKE THE SAUCE: In a small bowl, combine the soy sauce with the water, lemon juice, fish sauce and scallion.
MAKE THE DUMPLINGS: Very coarsely chop two-thirds of the shrimp and transfer to a food processor. Add 1 1/2 tablespoons of the cornstarch, the soy sauce, lemon juice, egg, lemon zest, salt and pepper and process until very finely chopped. Scrape into a large bowl. Cut the remaining shrimp into 1/3-inch pieces and add to the bowl. Mix in the jicama and scallions.
Dust a baking sheet with cornstarch. Lay 4 gyoza wrappers on a work surface; keep the rest covered with plastic wrap. Moisten the edges of the wrappers with water. Place a level tablespoon of filling on one side of each wrapper and fold in half to form half-moons. Press all around the filling to release any air pockets and to seal. Lift each dumpling by the sealed edge, transfer it to the baking sheet and press down lightly to flatten the bottom; the dumpling should be standing with the sealed side up on the baking sheet. Repeat with the remaining wrappers and filling.
Fill a wok or a very large skillet with 2 inches of water and bring to a boil. Line a double-tiered bamboo steamer with lettuce leaves and arrange half of the dumplings in the steamer without crowding. Cover and steam over moderate heat until firm and cooked through, about 12 minutes. Steam the remaining dumplings. Serve the dumplings right away, passing the dipping sauce at the table.