1. Heat the oil in a large saucepan and sauté the onion for 5 minutes or until softened.
2. Add the beef mince and cook for 5-8 minutes or until lightly browned.
3. Add the tomatoes, beans, Vegemite, tomato paste, chilli flakes and water. Stir to combine and simmer gently for 20-30 minutes or until thickened.
4. Meanwhile, place avocado in a bowl and use a fork to mash until smooth. Add lemon juice, diced tomato, salt and pepper and stir until combined. Set aside.
5. Preheat grill on medium.
6. Place Cornchips into an ovenproof dish. Top with chilli beef mince and sprinkle with cheese.
7. Cook under grill for 4-5 minutes, or until cheese is melted and golden brown.
8. Serve with avocado, sour cream and fresh coriander.