Drain and rinse the kidney beans in a colander to get rid of the dark sludge from the tins.
Put the onion slices into a bowl and pour over the vinegar. Leave to macerate for about 15 minutes.
Halve the cherry tomatoes and put them into a bowl.
When you are ready to assemble the salad, combine the drained kidney beans, macerated onions and halved tomatoes, tossing together well. Dress with the olive oil.