To make the salsa, rinse the beans and soak them overnight in cold water.
Next day, drain and rinse the beans, then put them in a saucepan with enough cold water to cover. Bring to the boil and cook vigorously for 10 minutes, skimming off any foam as necessary. Reduce the heat, cover and boil gently for 1 hour, or until the beans are soft but not falling apart. Rinse them under cold water, drain and leave to cool.
Put the kettle on to boil. Cover the tomatoes with boiling water and leave for 1 minute. Then skin and deseed them and chop the flesh into bean-size dice.
Place the beans and tomatoes in a mixing bowl and stir in the chilli, coriander, garlic, lime juice and 1 tablespoon of the olive oil. Season with salt and pepper, then set aside to marinate for at least 30 minutes.
Cut off the skin and any dark-coloured flesh from the tuna, then season. Heat a ridged, iron grill pan or nonstick frying pan over a high heat. Once hot, add the remaining oil. As soon as the oil is hot, add the fish and fry it for 1-2 minutes on each side until it flakes easily. Alternatively, to barbecue the fish, brush the barbecue rack or plate with a little oil and sear the tuna for 2-3 minutes on each side.
Serve the fish hot or at room temperature with the black bean salsa spooned over and garnish with lime wedges.
Variation: most supermarkets sell dried black beans, but if you cannot find them, substitute dried chickpeas instead, soaking and cooking them as above. You can also use well-drained and rinsed canned black beans or chickpeas, in which case omit steps 1 and 2.