French Onion Soup with Brie de Meaux and Croûtes

This is a classic among classics. A French onion soup can so often taste watery and underwhelming but this version delivers a meaty sweetness, balanced by a slug of vinegar and an even greater hit of Madeira. The croûtes and cheese are key, and here I have attempted to liven up the party and make the dish more hearty by using a soft Brie de Meaux. It works really well, either whisked to a thick and creamy spreadable mixture, or just cut and dropped in, as in this recipe, suitably stringy an