Melt the butter over medium high heat and sautee the onions until they begin to brown. Add the crawfish and creole seasoning and sautee for 2 minutes. Remember that most crawfish tail meat that is available in grocery stores is already cooked, so it is only necessary to bring the crawfish to temperature and infuse them with the flavor of the butter, onions, and seasonings.
Remove the mixture to a mixing bowl and fold in the bread crumbs. Allow the mixture to stand for 15 minutes or so before adding the egg. If you add the egg to a very hot mixture, it will scramble, so be careful! If I am in a hurry, I will put the mixture in the freezer to speed the cooling process. When the mixture is cool, add the beaten egg and combine well.
To construct wontons, place one wonton wrapper on work surface with one point facing you. Mound one teaspoon of the filling in the bottom corner, about 1/2" from the edges. Moisten the edges with egg white. Fold the top corner over the filling, making a small triangle. Bring side corners together, overlapping slightly. Fold the top of the wonton over the wonton filling tightly, pushing out any excess that may have been trapped when it was covered.
This is to ensure that the filling cooks completely and there is no residual oil trapped underneath the wonton wrapper. Firmly seal the edges of the wonton. Bring the side corners together overlapping slightly and pinch together firmly to seal. Place completed wontons on a large platter covered with wax paper. When you have filled all the wontons, cover the platter with a wet kitchen towel until you are ready to cook them. You can prepare these a day in advance and refrigerate them as long as you keep them covered with a wet kitchen towel.