Instructions: Cut the butter in small cubes and put in a bowl. Set the bowl in ice water.

(The butter should be very cold; otherwise it might melt too quickly and the sauce might break.)

Place shallots in a medium stainless-steel saucepan. Add the white wine and wine vinegar. Boil the liquid over high heat until it is reduced to a scant 1 tablespoon.

Just before serving, add the pieces of butter to the pan, 1 at a time, whisking well after each addition, until it has all been used. (The sauce should be fairly thick.) Add a little lemon juice to the sauce to taste.

Season with salt and white pepper.

If youre a purist, youll want to strain the shallots out of the sauce before serving. Some chefs prefer to leave the shallots in; if youre among them, you might also want to mix finely minced parsley or tarragon into the sauce. And some chefs like to bolster the sauce with a little creme fraiche just before serving it. Keep sauce warm until needed. This sauce can be made up to 1 hour in advance and kept in a bain marie over warm water.

HALIBUT:
Preheat oven to 500 degrees.

Season fillets well with salt and pepper. Cover fillets with mustard and place into a roasting pan.

Cook fillets at 500 degrees for 10 minutes.

Remove from pan to a plate and surround with beurre blanc. Serve.

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