For the vinaigrette, mix together lemon juice, vinegar, salt, pepper and approx. 1 teaspoon sugar, whip in the oil vigorously.
Wash the parsley, shake dry, roughly chop and stir in.
Put the beans in a colander, rinse with cold water and drain. Drain the tuna and tear apart with a fork. Peel the onions and cut into thin rings. Wash tomatoes and cut in half. Mix with the beans, onions and tuna into the vinaigrette. Let the tuna and bean salad stand for 30 minutes. Then season again with salt, pepper and vinegar.