Heat the oil in a large frying pan and fry the onion for 4-5 minutes, add the tomatoes, Walnuts and beans and cook for 2-3 minutes. Stir in the vinegar and most of the basil, season to taste.
Spoon over the toast and scatter with the remaining basil and some chopped Walnuts.
Cooks tip
Try adding some pan fried diced pancetta to the tomato mix for extra flavour, or swap the cannellini beans for chick peas.