Combine 1/4 of the chopped onion and one of the chopped cloves of garlic with the fresh coriander and 1 of the diced tomatoes.
In a medium bowl, mash the avocados with a fork and mix until mostly smooth. Add the onion/garlic/coriander/tomato mix and combine; add a few dashes of the lime juice and salt and pepper to taste. You can also add a bit of Tabasco if you prefer a little kick. Set guacamole aside.
To make the filling; over medium heat in a large frypan, put the rest of the onion and garlic in with the olive oil and and cook for 4-5 minutes, stirring to ensure thorough cooking.
Add the seasonings, mix thoroughly and then add the capsicum and mushrooms, cooking until the onions are translucent and the mushrooms are cooked. Add a small dash of lime juice for fun, give it a good mix in the pan and then set aside.
Warm the tortillas in a microwave for about 15-20 seconds, or heat them in the oven (low temperature) for a few minutes.
Scoop as little or as much of the veggie filling into the tortillas and serve with the lettuce, the two chopped tomatoes, cheese, sour cream and/or salsa on the side as a topping.