Taco filling (not following corporate instructions)

Old El Paso produces spicy mixes and salsa to go with Mexican food, e.g. tortillas, fajitas and tacos.

I bought the taco making kit, and then I realised they didn't mention having to cook tomatoes. o__O I found that a bit strange because I can't see how to professionally chop tomatoes, as a novice chef!

Ingredients [for two] [other than El Paso related things and the cheese]
- 300g chicken
- 1/2 onion
- 3 tomatoes

What I did to make, and with the taco filling instead was this:
1. Slice Onions [so not on their list of ingredients]
2. Chop up Tomatoes -- somehow.
3. Heat pan on fire, then add oil, and then stir-fry Onions.
4. Don't let Onions burn. When they look a little bit cooked, add small bite-sized pieces of Chicken. Let it brown a little.
5. Then add Tomatoes. Stir-fry a little.
6. Add the Old El Paso spicy mix -- not too much at one go cause it might be too hot, and 100ml of Water.
7. Let the water boil and simmer away until it disappears!