For the vinaigrette, mix lemon juice, vinegar, salt, pepper and approx. 1 teaspoon sugar, fold in the oil. Wash parsley, shake dry, chop coarsely and stir in.
Put the beans in a sieve, rinse with cold water and drain. Drain the tuna and tear with a fork. Peel onions and cut them into thin rings. Wash and halve the tomatoes.
Mix with beans, onions and tuna into the vinaigrette.
Let the bean salad steep for about 30 minutes. Then season again with salt, pepper and vinegar. Served with: flatbread.