THE COMBINATION of beef, mushrooms, and béarnaise sauce is a treat for any steak lover. This is based on the classic filet de boeuf Charlemagne, in which the fillet of beef is roasted, then sliced and reconstructed with a layer of cooked shallots and mushrooms (duxelles) between the slices. Just before serving, the roast is reheated and coated with the béarnaise. This classic presentation is only for experienced cooks, while my adaption is for everyone. With the addition of two more steaks, it can easily serve six. All accompaniments should be ready to serve before cooking the fillet steaks.