Fry the chopped onions in the olive oil, medium heat until translucent.
Add the fennel and celery, cook for five minutes, then pour 1 glass of water. When boiling add the potato, salt and peeper and cook until it’s cooked.
Blend all the vegetables until smooth.
Lightly fry for a few minutes the spring onions, add the fennel for another minute.
Line a tray with baking paper, spread the onions and fennel and place on top the fish a few centimeters apart.
Brush some olive oil and add salt and pepper.
Cover with another backing paper, and seal with aluminum foil.
Cook in a hot oven, 200 degrees Celsius for 18 minutes.
Pour the warmed vegetable sauce in a deep serving plate and place the fish on top. Sprinkle with some crushed red peppercorns.