Wash the fish fillet, dab dry and cut into cubes. Sprinkle with lemon juice (keep one tablespoon) and leave to stand. Clean, wash and slice the celery.
Pluck the young inner leaves and put them aside. Wash and cut the tomatoes into six. Peel and finely chop the onion. Put the onions and oil in the microwave oven, cover and fry until transparent at 600 watts for 1 1/2 minutes.
Add the celery (without leaves), bay leaf and 1/8 litre stock, cover and cook at 600 watts for three minutes. Add tomatoes and cook at 600 watts for another two minutes. Add fish cubes and remaining stock and cook at 600 watts for four to five minutes.
Cover and leave to stand in the mould for five minutes. Peel the garlic and press through a garlic press to make mayonnaise. Mix with remaining lemon juice and season with salt and pepper. Wash the dill, dab dry and chop finely.
Stir the prawns and celery greens into the hot soup, season to taste with salt and pepper, sprinkle with dill and serve with the aioli. Serve with fresh baguette.