Broiled Mussels with Onions and Mussel-Flavored White Wine Sauce

This dish is my own little invention, but each of its components is based on French classic cooking. The mussels are steamed open exactly as for moules à la marinière, a hollandaise-type sauce is made with steaming liquid from the mussels (a derivative of a classic sauce vin blanc, in which reduced fish broth is whisked with the egg yolks into a sabayon before the butter is added; described in the Sole Marguéry recipe), onions are sweated and caramelized a