How to Make It
Put the flour in a medium bowl. Add the water in a steady stream, stirring until a raggy dough forms. Turn the dough out on a work surface and knead until smooth, about 5 minutes. Sprinkle the dough with flour, cover loosely with plastic wrap and let stand at room temperature for 15 minutes.
In a large skillet, cook the watercress over moderately high heat until wilted, about 2 minutes; squeeze out any liquid. Transfer to a large bowl and let cool. Add the tofu, scallions, garlic, water chestnuts, sesame oil, egg, salt and white pepper. Using your hands, knead the mixture to form a coarse paste.
Line a baking sheet with wax paper and dust with flour. Quarter the dumpling dough. On a lightly floured work surface, roll each piece of dough into a 12-inch rope. Cut each rope into 12 pieces and roll into balls; sprinkle with flour. Working with 5 or 6 balls at a time, roll to 3 1/2-inch rounds. Brush the excess flour off of the rounds. Spoon 1 tablespoon of the filling onto the center of each round. Bring up the sides of the wrapper; press and pleat the edges to seal in the filling. Lift each dumpling by the pleated edge, transfer to the baking sheet and press down lightly to flatten. Repeat with the remaining dough.
In a very large nonstick skillet, heat 2 tablespoons of the oil. Arrange half of the dumplings in the pan, pleated edge up. Cook over high heat until the bottoms are lightly browned, about 2 minutes. Add 1/2 cup of water to the skillet, cover and cook until the water is evaporated and the dumplings are cooked through, about 5 minutes longer. Uncover and cook until the bottoms are well browned, about 1 minute longer. Transfer the dumplings to a plate. Cook the remaining dumplings in the remaining oil and serve.