Clean, wash and cut the spring onions into rings. Clean, wash and halve the mushrooms. Dab fillet dry and cut into 8-10 pieces.
Heat the oil in an ovenproof pan or roasting pan. Sauté the fillets in it for about 5 minutes. Season with salt and pepper and remove.
Fry the mushrooms in portions in hot frying fat for about 5 minutes. Season with salt and pepper. Dust with flour and sauté briefly. 1⁄2 Stir in l water, cream and stock, bring to the boil. Season sauce with salt and pepper.
Place the meat in the mushroom sauce. Spread spring onions on top. Cook in the preheated oven (electric cooker: 175 °C/circulating air: 150 °C/gas: level 2) for 20-25 minutes.
Wash the chives and chop finely. Cut herb butter into slices. Melt on the fillets. Sprinkle chives over them. Delicious with baguette or new potatoes.