Wash and drain the watercress and pluck the leaves from the stems. Put something aside for garnishing. Roughly chop the remaining leaves. Put everything in a cool place. Peel onion and garlic.
Finely dice both. Clean, wash and cut the zucchini into large pieces. Peel and wash the potatoes and cut them into large pieces. Heat 2 tablespoons of oil in a saucepan. Fry onion, garlic, zucchini and potatoes for 3-4 minutes.
Deglaze with broth and cook covered for about 15 minutes. In the meantime heat 2 tablespoons of oil in a large, coated pan. Fry the slices of bread on both sides until golden brown. Remove the finished slices of bread, season with salt and drain.
Bring 1 litre of water to the boil in a wide, flat pot. Season with salt and 5 tablespoons of vinegar. Beat the eggs in 4 cups and carefully slide them into the vinegar water. With a spoon, place the egg white over the egg yolk.
Cook at low heat for about 4 minutes. Add chopped watercress to the soup, bring to the boil briefly, puree finely and season to taste with salt, pepper, lemon juice and sugar. Arrange the soup with bread chips and eggs and garnish with watercress.