SAUCE: Discard the cooking oil and analyze the pan residue. Remove any unappealing bits. Deglaze the pan with the white wine. Use a wooden spoon to soften and dissolve any brown residue on the bottom of the pan. Increase the temperature and simmer the wine until the aroma of raw alcohol has cooked off and the volume has reduced by half. Stir in the mustards, horseradish, rosemary, and any juices that have collected under the halibut and bring just to a simmer. Gradually stir in the butter, a few pieces at a time, until just melted (or add enough olive oil to make it saucy); do not boil after the butter has been added. Taste and adjust the seasoning with salt and pepper as necessary.