Combine all dressing ingredients in a blender and blend until smooth.
Peel the onion, cut into thin slices, put in ice-cold salted water for 15 minutes.
Cut the tomatoes into large slices.
Dry the onion in a towel and mix with the tomatoes. Drizzle with dressing, stir, sprinkle with zest and black pepper, serve immediately. We recommend serving this salad with toasted bread.