Open and peel the pea pods. Peel and finely chop the onion. Heat 1 tablespoon of fat in a pot. Fry the onions in it until transparent. Add peas and steam briefly. Deglaze with the broth and bring to the boil.
Simmer covered over a low heat for about 20 minutes. Peel and wash the potatoes, cover and cook in boiling salted water for about 15 minutes. Wash fish fillets, dab dry and cut in half lengthwise.
Heat white wine with 3/4 litre water, bay leaf, peppercorns and 2 teaspoons of salt. Wash lemon hot and cut into slices. Add fish slices and lemon wedges to the hot, but not boiling broth and let it simmer at low heat for 8-10 minutes.
Take some peas out of the stock, puree the rest with the cutting stick of the hand mixer. Add whole peas and cream and bring to the boil. Season the sauce with salt, pepper and nutmeg. Wash the chervil, dab dry and pluck off the leaves, except for a few stems for garnishing, and add to the sauce.
Keep the sauce warm. Drain the potatoes. Heat 2 tablespoons of fat in a pan. Fry the potatoes in it until golden brown all around. Heat the remaining fat and lemon pepper. Drain the fish slices and serve with the potatoes and the pea sauce.
Sprinkle fish with lemon pepper butter and serve garnished with the remaining chervil.