Put some oil in a hot pan and roast the onions until golden brown. Add mustard and tomato paste and roast a little, extinguish with brandy, simmer briefly, pour in white wine and simmer gently. Add peppercorns and sour cream, season, let simmer a little more. If the sauce is too thick, add a little vegetable stock.
Spread a thin layer of oil on a hot grill pan and sear the seasoned fillet pieces and continue to fry slowly until they have reached the desired cooking point.
Roast the toast and place on a plate, spread the lettuce leaves on top and place the fillet pieces on top. Now spread the sauce over it with a spoon, sprinkle some freshly ground pepper on top.