Beef Tenderloin “à la Ficelle” with Roasted Beets, Bone Marrow Crouton, and Horseradish Sauce

I call this “a la Ficelle,” beef on a string—which often denotes meat cooked by turning on a string over a fire—because in France, one often poaches a piece of beef in a pot of stock that’s already cooking; in order to remove it easily one would tie a string to the meat, hanging over the side of the pot. and then you used the string to pull it out of the pot. This is tenderloin very gently poached and served with a horseradish sauce, which is not too diffe