Heat the oil in a pan over a medium heat. Add the onion and garlic and cook gently for 8 minutes until softened, taking care not to let them burn.
Add the spices and cook for another 3 minutes until aromatic, then add the black beans, tomatoes and 250 ml of water and bring to the boil. Reduce the heat and simmer for about 20 minutes or until almost all of the water has evaporated. Season with salt and pepper to taste.
Heat the tacos or tortillas. Serve the black beans on top with the guacamole and salsa, then sprinkle with fresh coriander leaves, chilli flakes and lime wedges.