To make the filling: Combine the shrimp, scallops, pork, ginger, cilantro, shallots, vinegar, sesame oil and egg whites in a food processor. Season with salt and pepper, add the corn starch and lemon juice and pulse to combine-- I like my fillings to have a little texture. (Alternatively chop the shrimp and scallops finely then add the pork, ginger, cilantro and vinegar and chop together. Transfer the mixture to a bowl and rapidly mix in the vinegar, sesame oil, egg whites, seasoning, corn starch and lemon juice.)
To make the shumai: Hold a wonton wrapper in your hand. Spoon 1 tablespoon of the filling into the center of the wrapper (rinse the spoon in cold water so the filling doesn't stick). Gather the edges of the wrapper up around the filling and squeeze the sides together with your fingers forming a little pleated cup. (The sides will naturally pleat, leaving the filling slightly exposed.) Tap the dumpling on the table so the bottom is flat and it stands upright then set aside on a baking sheet. Repeat with the remaining wrappers and filling. Top each shumai with a pea. (You can freeze any leftover filling.)
Lightly oil the bottom of a bamboo steamer then line it with the cabbage leaves. Stand the dumplings in the steamer in a single layer taking care that they don't touch one another. (A 10-inch steamer will accommodate 12 shumai). Bring about 2 inches of water to a boil in a wok. Set the bamboo steamer in the wok, cover it with the bamboo lid and steam until the shumai feel firm, 10 to 12 minutes. Serve the shumai immediately garnished with green onions and accompanied by soy sauce or dipping sauces of your choice.