Preparation To make the soy-mustard sauce, combine all the ingredients in a small bowl and mix well. Set aside. To make the filling, cut the shrimp into bite-size pieces, then roughly chop until the shrimp become almost paste-like but with some small pieces visible. You can also use a food processor, which will give it a smooth texture, but I prefer the more coarse texture you get from chopping by hand. Transfer the shrimp to a bowl and mix with a spatula until the shrimp become very sticky. Add the scallions, water chestnuts, and mushrooms to the shrimp and mix well. Then add the following ingredients, mixing well between each addition: lard, sake, ginger juice, sesame oil, and egg white. Finally, mix in the salt, pepper, and potato starch. Meanwhile, place the baby bok choy in a bowl and cover with cold water; let it sit for 10 to 15 minutes. To assemble the shumai, lay a wonton skin on the counter (cover the remaining skins with a damp towel to keep them from drying out). Place 1 tablespoon of the filling in the center of the wonton skin. (Soak the spoon in water while you’re assembling a shumai. This will make it easier to transfer the filling to the wonton.) Hold the wonton skin with the filling with your fingers and gently press the skin to form a small cup. Set the shumai on a cutting board and turn it clockwise while carefully pressing the sides together, creating a tighter cup. Wet a finger with water and use it to gently smooth the top of the shumai. Repeat with the remaining wonton skins and filling. To cook the shumai, heat a stove-top steamer over high heat, bringing the water to a boil. Brush the bottom of the steamer basket with a small layer of the vegetable oil and add the shumai, leaving 1 inch of space between each. Cover and cook for 3 minutes, then drain the bok choy and add to the steamer. Cover and cook for 3 minutes longer, or until the shumai are cooked through. Remove the bok choy from the steamer and gently squeeze inside a towel to remove excess moisture. Top with a pinch of salt. Place each bok choy in the center of 4 small plates and arrange 6 shumai around it. Garnish the plates with a pinch of black sesame seeds and serve with the soy-mustard sauce on the side.