Warm oil in a skillet over medium heat, add chicken and cornstarch and cook - chicken 4- 5 minutes each side - stirring constantly.
Remove chicken from pan and cool.
Keep oil in pan, keep hot.
Prepare stir fry sauce by mixing together the soy sauce, brown sugar and rice vinegar in a small bowl.
When chicken is cool mince or cut into tiny pieces.
With pan still on heat, add another T of vegetable oil - add chicken, garlic, onions, mushrooms and scallions to pan. Add stir fry sauce to pan and saute the mixture for a couple of minutes.
Then serve it in the lettuce cups.