Cut four thin slices crossways from the hat of the raw mushroom. Fry the onion in some butter, add the sugar, vinegar and veal fond and allow to simmer until you acquire a slightly syrupy sauce. Brush the olive oil over the beef cheek and grill it for 3 to 4 minutes. Season with salt and pepper. Place the stewed red cabbage next to the grilled beef cheeks on plates and slide the mushroom slice between them.