Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Line baking tray with baking paper. Wash fillets, dab dry, season with salt and pepper. Heat clarified butter in a large pan.
Brown the fillets in it all around. Take them out, put them on the tray and roast them in the hot oven for about 15 minutes. Put the pan aside.
Rinse the beans in a sieve and let them drain. Clean and wash the chard. Cut off the stems and cut into large pieces. Clean mushrooms, rinse briefly and cut into thick slices.
Heat the oil in a frying pan in the frying fat. Fry the chard stems and mushrooms for 3-5 minutes. Season with salt and pepper. Clean, wash and chop the spring onions. Add and fry briefly.
Coarsely chop the chard leaves. Crush the beans slightly with a fork. Add both with 5-7 tablespoons of water, bring to the boil and simmer for 2-3 minutes. Season to taste with mustard, salt and pepper. Remove the fillet from the oven, cut into slices and arrange with the vegetables.