Clean and wash the frisée salad. Clean and slice the radicchio and carefully wash the resulting roses as well. Peel and finely chop the onion. Mix vinegar, mustard, salt and diced onion.
Add olive oil little by little. Arrange the salad on plates and sprinkle with the vinaigrette. Wash, clean and halve the tomatoes. Heat 1 tablespoon of oil in a frying pan. Sear the tomatoes over a high heat, remove.
Cut red onions into eighths and add to the salad. Wash the prawns, dab dry and sprinkle with lemon juice. Heat 1 tablespoon of oil in a frying pan and fry the prawns on both sides for about 1 minute.
Add cherry tomatoes and prawns to the salad and sprinkle with pepper.