Beurre blanc is a classic French butter sauce, very delicate and flavorful, often used as an accompaniment to mild fish. It's lighter and easier to make than a hollandaise-type sauce - but it doesn't hold well, so it should be made at the last minute for best results. The reduction, however, can be made hours in advance, which allows the beurre blanc to be whipped together at serving time. The finished sauce can be held in a warm thermos for a couple of hours.
Place the orange juice and shallots in a small saucepan and cook over medium heat until the juice has reduced to about 3 tablespoons. At this point, the reduction can be set aside at room temperature until just before serving.
If you've made the reduction in advance, reheat it over medium-low heat until very warm. Otherwise, if you've skipped the waiting period, reduce the heat to medium-low and whisk in 1 piece of butter, stirring constantly. As soon as the butter melts, whisk in another piece, stirring constantly. Continue in this fashion until you've incorporated all the butter. Don't let the sauce get too hot, or the butter will liquify and the sauce won't emulsify. The trick is to keep moving the pan on and off the burner to control the heat. With each piece of butter added, the sauce will thicken.
Once all of the butter is incorporated, add the zest, then salt and pepper to taste. Serve immediately or transfer to a wide-mouthed thermos.