Add cold chicken, beans, chili's, chili powder, water, veggies and about 1/2 of the chopped onion to a deep frypan that has a cover. Bring to a simmer for about 5 minutes, stirring to mix all ingredients, then cover and slowly simmer for another 10 min.
Remove lid and spread cheese over the chicken mixture in the pan. Cover pan and turn off the heat.
Warm any type of taco/burrito shells (my family is partial to flour) in fry pan, microwave or however you normally do it. I warm mine in a pan that has a cover and just bring the covered pan to the table.
Going back to the chicken, when you remove the lid, the cheese should be melted. Sprinkle the rest of the chopped onion, tomatoes or salsa or both and the lettuce evenly over the melted cheese and bring to table with big spoon. Everyone gets to make their own taco. One spoonful gives you everything, filling cheese and toppings. Or you can be traditional and let everyone add their toppings at the table.
You can be creative with this, add green peppers, cilantro, jalepenos, whatever your family likes. I serve with low fat sour cream and sometimes guacamole.