The beer is a typically Flemish as the dish. In his Carbonade Flamande, Escoffier suggests either an old Lambic or a Stout. These are extremely different styles, and a Flemish Old Brown falls between them at equal distances. It is not tart as the one or as burnt-tasting as the other in its dark malt character. Its lactic acidity seems perfectly to tenderize the beef, and that Montilla character adds piquancy to the cooking liquid. While preparing this dish, I enjoy a glass of the beer, which serves as a perfect pick-me-up in the late afternoon. Then I put my Carbonade in the oven let my appetite become aroused while the dish cooks slowly for three hours, in the time for a wonderful dinner.
Like Lambic in the making of Gueuze, the Old Browns traditionally are blends of young and old beers. This is also true of several similar beers with a redder colour that are made mainly in West Flanders.
This blending can be a means of creating a secondary fermentation, but it is also a way of combining and balancing flavours. In the days before refrigeration, when no one brewed in summer, a stock would be laid down for those months. By the autumn, the last of this beer would have become winey and acidic, and might be blended with, fresher, sweeter, new beer to enliven its flavour. Eventually, “old” beers were produced purely as an ingredient for blending. While the Lambic and West Flanders brewers traditionally use wooden vessels for aging and blending, iron or steel are usual in the East Flanders breweries.
From "The Great Beers of Belgium" by Michael Jackson