Parboil scallops in water that does not quite cover. Add vinegar. Keep water just below boiling. Cook scallops just until they turn white. They cook quickly and overcooking will toughen them. Drain and reserve the cooking liquid.

Quarter the scallops if large. To the cooking liquid, add enough water to make 1-1/3 cups total.

Melt butter in a double boiler. Add flour, stir, and blend. Add scallop liquid gradually, continuing to stir and cook over low heat. Add mayonnaise slowly, as soon as sauce thickens. Add scallops, thyme, and salt. Heat through but do not allow water-under to boil. Serve on toast or in toast cups.