Peel the onions and cut into rings. Cut peel off the grapefruits and cut out the segments between the separating membranes. Set some walnut halves aside and chop the rest. Mix balsamic vinegar gradually with oil. Season with salt, pepper and sugar to taste. Rinse lemon balm and set aside a stalk for garnish. Strip remaining leaves from the stems, cut into strips and add to the dressing.
Mix all ingredients together and let stand for about 30 minutes. Serve garnished with the remaining lemon balm and walnuts.