Clean, wash and cut the celery into small pieces. If necessary, steam in boiling salted water for 1-2 minutes. Drain and let cool down
Mix vinegar and mustard. Season with salt, pepper and sugar. Whip the oil into it
Peel onions and cut them into rings. Wash and halve the tomatoes. Cut olives in half if necessary. Clean and wash spring onions and cut them into rings. Mix prepared salad ingredients and mustard sauce, let stand for about 30 minutes
Remove mackerel from the skin with a sharp knife and cut into strips. Clean and wash the lettuce leaves and pluck them smaller. Season celery salad again. Serve with mackerel on salad