Preheat your griller. If you have a pizza stone, place it under the griller while it preheats.
In a food processor, combine the avocados and coriander. Puree while adding lime juice and salt to taste. Cover and set aside.
Slice the garlic clove in half and rub the cut side onto the tops of the pizza bases for flavour. Brush both sides of the bases with olive oil. Spread the avocado mixture thickly over the top of each base, then arrange chicken and tomatoes on top. Sprinkle with cheese and season lightly with cayenne pepper. Place pizzas on a baking tray if you do not have a pizza stone.
Grill for about 5 minutes in the preheated griller or until the cheese has just melted and the crust is lightly toasted.