Preheat oven to Gas Mark 4, 180°C (350°F). Heat a little oil in a large frying pan over a high heat, add in half the beef, cook for approximately 5 minutes or until browned. Remove to a plate and repeat with the remaining beef. Add a little more oil to the pan and turn the heat down to medium. Add the onion and garlic and cook for 5 minutes until soft but not brown. Add the cumin, coriander, chilli and chilli flakes. Stir and cook for a minute or two. Add back in the beef along with the pasatta and the water and bring to the boil. Season with a little salt and black pepper. Reduce the heat and simmer, uncovered, for 10 minutes. Add in the beans and continue to cook until beef mixture thickens. This will take another 10-15 minutes. Spread half the corn chips into a large oven proof dish, top with half the beef mixture and sprinkle with half of the cheese. Repeat with the remaining corn chips, beef and cheese. Bake, uncovered, for 10-15 minutes until heated through. Combine all the salsa ingredients together in a bowl, taste and season. Combine the avocados with the lemon juice and olive oil. Season. Spread a little of the salsa and avocado and a drizzle of Greek yogurt over the warmed nachos. Bring the nachos and remaining salsa, avocado and yogurt to the table and let everybody tuck in.