Having cooked your steak, take it out of the pan and keep it hot. Throw away the fat without washing the pan. Melt the butter in the same pan. Soften the chopped shallots in the butter, allowing them to cook for about 2 minutes.
Pour in the vinegar and allow to reduce by half. Season with salt and pepper and add the chopped parsley and the blood which has run out of the meat.
Arrange the steaks on hot plates and cover them with the shallot sauce.