Adjust the seasoning if necessary. Use or place in a container, cover and store refrigerated for up to 2 days.
To form the shumai, have a bowl of water handy. Hold 1 wonton wrapper in the palm of your non-dominant hand. Cup the hand and place 1 heaping tablespoon of the mousse in the centre of the wrapper. Bring the wrapper up around the filling, pressing it to adhere to the filling and pleating as you go. Continue around the filling. There will be 6–8 pleats and the filling will be exposed. Tap the dumpling against the work surface to flatten the bottom. Repeat with the remaining wrappers and filling.
Put the scallions on a platter. Add the sesame seeds and combine. With a wet palm, press down on the shumai, flattening them to a thickness of about ½ inch. Press the ‘open’ top side of the shumai into the scallion mixture.
Line a large plate with paper towels. Heat a large skillet over medium heat. Add 2 tablespoons of the oil and swirl to coat the pan. When the oil is hot, carefully add half the shumai to the pan coated side down and cook until golden, turning once, for 1 ½ – 2 minutes per side. The tip of a paring knife, when inserted in the shumai, should emerge hot.
Transfer the shumai to the kitchen paper to drain. Cook the remaining shumai with the remaining 2 tablespoons oil. Transfer to a platter, sprinkle with the reserved scallion greens and serve.