Fry the chopped onion in olive oil until soft. Add soft brown sugar, wine vinegar and Cajun seasoning. Cook for 1 min then add the mixed beans and chopped tomatoes with garlic. Simmer for 10-15 mins until the sauce has thickened then season.
Scatter handful grated cheddar onto tortilla chips. Microwave on High for 1 min until cheese has melted. Serve alongside the dip (top with some chopped avocado if you like). Make ahead and reheat dip before guests arrive.