In a small fry pan over low heat, cook the onions until they turn a caramel color, this can take up to forty-five minutes. In a small saucepan, cook the cherries with the sugar and the water, adjusting each to taste. Cook the cherries for twenty minutes until they soften and the natural pectin begins to thicken the mixture. Add the mustard, vinegar and onions and check the seasoning. If the mixture is too tart, add more sugar or honey, if too sweet, add more vinegar. The moustarda can be made ahead of time and served warm or cold, it will keep for several days in the refrigerator.
Season the Chateaubriand with salt and pepper and sear in the clarified butter in a hot pan. Place the pan in a hot (400 degree oven) and roast for 20 minutes or until an internal read thermometer registers 155 degrees. Remove the roast from the oven and the pan and let rest for ten to fifteen minutes.