Start by cutting up some good stewing beef—I prefer chuck—into serving pieces, either cubes or (as the older recipes call for) small, thick slices. Brown each piece well in lard (or butter) and set aside. In the same skillet, sauté lots of onions, about half as much as you have beef by weight (or even more if you like onions) until they have softened.
Now layer a Dutch oven or braiser with half the beef, then half the onions. Season well with salt and pepper. Repeat with the rest of the beef and onions. Then take a ladleful of good, strong beef stock and deglaze the skillet in which you browned the beef and onions, and pour it into the pot. Top off with some good beer or ale—preferably Belgian ale, of course—enough to cover the beef completely. Nestle a bouquet garni, made with a bay leaf, a spring of fresh thyme and a sprig of parsley, among the beef pieces. Most recipes also call for adding a pinch of brown sugar at this point, just enough to balance some of the bitterness of the beer but not enough to add any actual sweetness to the dish.
Bring the pot to a simmer on top of the stove, cover, and place in a moderate oven (180°C, 350°F) to braise for 2-1/2 or 3 hours, until the beef is fork tender. (The dish can be made ahead up to this point.)
A few minutes before serving, remove the pot from the oven and, keeping at a slow simmer on top of the stove, thicken the sauce with a slurry of corn starch and water, together with just a few drops of vinegar, if you like. (Again, not too much—just enough to add a tiny bit of ‘zip’ but not so much that you can actually taste any sourness.) Let the dish simmer for a few minutes and serve hot, accompanied by pommes frites, mashed or steamed potatoes or buttered noodles.
NOTES: As with many classic dishes, you will find many variations of carbonnade à la flamande. Not all recipes calls for the final fillip of vinegar, some call for a dollop of mustard. Some versions will have you add some lardons at the start, some add mushrooms—at which point the dish begins to resemble a boeuf bourguignon made with beer rather than red wine. The herbs and spices that go into the bouquet garni can vary; personally, I like adding a few cloves, while some recipes call for some allspice or nutmeg. And some recipes thicken the dish not with cornstarch but a beurre manié or—believe it or not—with some gingerbread (pain d’épices) that is either crumbled or placed on top of the beef to braise along with it. Of course, as the dish cooks the gingerbread breaks up and melts into the braising liquid, providing a nice liaison and an old-fashioned flavor.
One crucial choice will be, of course, the beer or ale, which can really transform the character of the dish. Needless to say, a good Belgian ale, perhaps of the Lambic or Trappist variety, would really be the best choice. But any beer or ale with good character, particularly amber ales, will work. (So will your average lager in a pinch, but the dish will inevitably lack some ‘oomph’.) If you want a really interesting taste, you can do what the Irish do and use stout, I which case (if you add some carrots along with the onions) you will be making a typical Irish dish called Beef in Guinness.
I am not sure of the origins of carbonnade—the word actually refers to cooking over coals—but it is surely very, very old. Some of the older variations that call for spices like nutmeg, the use of bread as a thickener and the sweet-and-sour flavoring (I suspect the vinegar was probably originally verjuice) are all quite typical of Medieval European cuisine.