For the vinaigrette, whisk lemon juice, vinegar, salt, pepper and honey. Fold in the oil. Wash parsley, shake dry, pluck leaves from the stalks, chop and stir into the vinaigrette.
Drain the beans in a sieve and rinse with cold water, drain well and mix with the vinaigrette. Peel the onions and cut into rings. Wash and halve the tomatoes. Mix the tomatoes, onion rings and tuna into the beans.
Season salad with salt, pepper and vinegar.