Heat the olive oil in a deep saucepan or pot. Add the onion, garlic, green chillies and bay leaves. Sauté until soft. Add the cumin and cook for a further minute.
Add the mince and brown. Add the tinned tomatoes with their juices, the drained beans, brown sugar and a good pinch of salt. Fill one of the empty tins with water and add this to the pot. Give the mixture a stir and leave to simmer with the lid on for 45 minutes.
Stir the mixture. Leave to simmer for a further 10-15 minutes with the lid off until most of the liquid has evaporated and the mixture has thickened. Remove the bay leaves and check the seasoning.
To make the sour cream combine the sour cream, coriander and lime juice in a small bowl and mix well. Keep in the fridge until just before serving.
Crisp up the taco shells in the oven as per the manufacturer's instructions. Pop a little fresh lettuce into each shell followed by a generous spoonful of chilli con carne. Top with lashings of sour cream and grated cheese. Serve immediately.