For the puree, peel, wash and roughly dice the celery. Wash the lemon hot, dry and finely grate the peel. Squeeze the juice out of half. Put the celery and lemon juice in enough salted water, bring to the boil and cook for about 20 minutes.
Preheat oven (electric cooker: 200 °C/circulating air: 180 °C/gas: see manufacturer). Wash and dry organic orange and rub the peel. Mix with the lemon peel. Wash the fish fillets, dab dry and place (skin down) on a baking tray lined with baking paper. Sprinkle the fillets with 2 tablespoons of oil. Sprinkle with lemon peel, season with salt and pepper. Cook in a hot oven for 10-15 minutes.
For the sauce in the meantime peel the clementines and cut them into pieces. Squeeze both oranges. Roast the fennel seeds in a pan without fat. Pour in orange juice and bring to the boil. Stir the starch with 2 tablespoons of cold water until smooth, stir into the sauce and simmer for about 1 minute. Add the mandarin orange pieces. Season sauce with salt and pepper and keep warm.
Peel garlic and chop finely. Drain the celery. Heat milk, 2 tablespoons butter and garlic, season with salt. Add celery and mash. Keep warm.
Wash, clean and finely slice the fennel. Mix with some oil and season with salt. If available, wash and chop the fennel greens and add to the sauce. Fish with fennel, sauce and S