To the chicken, add the onion, cilantro, shaved corn, tomatillo sauce and 2½ cups of grated cheese, salt and pepper. Using a wooden spoon, or your hands, mix ingredients together. Taste and adjust seasoning by adding more salt or pepper, if necessary.
Stuff the roasted chilis with the chicken mixture and top with remaining 1½ cups of grated cheese. Put finished chilis on a sheet tray. If you are doing this the day ahead cover tightly with plastic wrap and store in fridge.
Bake at 350° F for 20-30 minutes, until the chilis are hot and the cheese on top has melted. If you made them the day ahead, allow more cooking time in the oven or take them out of the fridge and let them come to room temp, before putting them in the oven.