Heat the oil in a large, deep frying pan over medium heat. Add the onion and garlic, and cook, stirring, for 4 minutes or until soft.
Add the mince. Cook, stirring with a wooden spoon to break up any lumps, for 5-6 minutes or until mince changes colour. Add the tomato and seasoning, and stir until heated through.
Preheat oven to 180°C. Place 2 tortillas over the base of a 2.5-3L (10-12 cup) capacity ovenproof dish. Top with half the mince mixture. Continue layering with 4 tortillas and the remaining mince mixture, finishing with tortillas. Place the avocado, in a single layer, on top. Top with the remaining tortillas. Spread with the sour cream. Top with the cheddar. Bake for 35-40 minutes or until golden. Serve.
