Roll the fillet pieces in flour and fry them in a pan with the olive oil and butter for about 3 minutes on all sides. Add the pickled onions, the sage leaves and the balsamic vinegar and simmer everything over a medium heat for 10 minutes. Season to taste with salt and pepper.
To prepare the sauce, remove the meat and onions and keep warm. Add the gravy and reduce a little. Pour the sauce into a tall container and whip together with the cold butter with a hand blender until frothy. Pour the finished sauce over the meat, grate the parmesan and serve immediately.