Peel and finely dice the onion and mix with 50 g butter. Season with salt and pepper. Form into a roll, wrap in aluminium foil and chill. Wash and peel the asparagus and generously cut off the woody ends.
Peel and wash the potatoes and cook them in salted water for about 20 minutes. Grate cheese. Dab meat dry. Fry all around in hot oil. Season with salt and pepper. Place the fillets on one half of a baking tray.
Drain the potatoes. Add 2 tablespoons of butter and toss well covered (the potatoes may crumble a little). Place the fillets on the tray next to the fillets and sprinkle with cheese. Bake in a hot oven (electric cooker: 175 °C/circulating air: 150 °C/gas: level 2) for about 20 minutes.
Cook the asparagus in boiling salted water covered with 1 teaspoon sugar for about 15 minutes. Wash and chop the herbs or cut into small rolls.
Foam 50 g butter, remove from heat, add herbs.
Drain the asparagus. Cut the meat open, spread onion butter in slices on top. Serve with potatoes and asparagus. Drizzle herb butter over the asparagus.