Brush the potatoes well under running water. Cook in water for about 20 minutes
Peel and finely chop the onion. Cut all olives into fine slices. Mix vinegar with 4 tablespoons of water, some salt, pepper, onion and pesto. Beat in olive oil vigorously
Drain potatoes well, peel if necessary. While still warm, mix with olives and pesto vinaigrette, up to 1 tsp. Leave everything to stand for about 1 hour
Wash and halve the tomatoes. Fold into the potato salad. Season the salad again with salt, pepper and possibly vinegar
Wash parsley, shake dry, pluck off leaves. Stir the sour cream until smooth and add it to the salad as a blob. Garnish with remaining pesto and parsley