After paring well, the fillet is seared in butter on all sides (in an appropriately large roasting pan). While frying the spring onions (cut into rings) are added. If the meat is golden brown all around, it is properly seasoned. Add the garlic and after 5 minutes more searing time, deglaze with veal stock.
The mushrooms are cleaned and halved / quartered depending on their size. You will now take a seat next to the fillet and will be boiled down. If the stock is largely reduced, turn the heat down a little and add the red wine. Put the lid on and let it simmer for 10 minutes.
Now get the fillet out and wrap it twice in aluminum foil. Finish the resulting roast stock in the cauldron with the remaining ingredients for the sauce. Let it simmer for 5 minutes at the highest level. If the sauce does not have enough binding, then add a tablespoon of cornstarch (mixed with water). Season again to taste and put the fillet in slices so that it gets more heat.
Fill the preheated plates with the desired side dishes. Then put the meat with the mushrooms on top. Spread the sauce "generously" on the plate.