In a food processor, add the roughly chopped onions, scallions, garlic, water chestnuts and pulse until finely chopped. Place in a large bowl.
Add the shrimp in the empty food processor and pulse until it's finely chopped but still has texture. (The reason we chop the shrimp separately is so that we can control how finely the shrimp is chopped).
Add in the shrimp, pork, salt, pepper, fish sauce, soy sauce, egg in the mixing bowl with the chopped vegetables and mix with hands or a spatula. To check if you're satisfied with the flavor of the filling, do a taste test by adding some oil in a pan and pan frying about half a tablespoon of filling. Then adjust filling as needed by adding more salt, soy sauce, or fish sauce.
Once filling is ready, fill a piping bag with the filling if you have one. An alternative would be to use a ziplock bag and cut one of the bottom corners to create your own piping bag. You can also use a spoon to scoop the filling and place it directly on the wrapper (about 1 - 1 ½ tablespoons).