Clean, wash and cut the celery into pieces. Peel onions and cut into rings. Clean, wash and halve the tomatoes. Drain the olives and possibly cut them in half as well. Clean, wash and cut spring onions into rings.
Mix vinegar and mustard and season with salt, pepper and sugar. Whip the oil into it. Mix all salad ingredients with the mustard sauce and let it stand for about 30 minutes. Season to taste again. Remove mackerel from the skin with a sharp knife and cut into strips.
Serve the salad together with the mackerel strips on salad leaves as desired.