This is more popularly known as beef carbonade, carbonade of beef, or carbonade à la Flamande. It is less popularly known as Vlaamse stoverij, which is probably its more accurate name given that it is Flemish beef stew, and that's well, Flemish. Or Dutch, if you want to be technical; Dutch and Flemish are practically the same language. In fact, the more that I think it about it, the more bizarre it is that people call it beef carbonade. Carbonade comes from the Italian carbonata, which means to cook over hot coals or to grill, whilst this is a braised dish. Quite frankly, it's the culinary equivalent of the Schengen agreement.
Being a traditional dish there is no defined method for it. Every family has its own variations and its own secret ingredients, from gingerbread, to redcurrant jelly, to vinegar. The general agreement is that it uses beef, dark Belgian beer, onions, and something to make it a little sweet and a little sour. And it takes a long time to cook. (Please don't let that put you off. It takes a long time to cook, not to prepare. I played an epic game of Scrabble whilst this was on the hob.) I would have called my auntie, the one who's Belgian, for her recipe, but the phonecall would have taken longer than the cooking time, so I used my best judgement and came up with this. There was sufficient for two of us, and we ate it with mustard-y mashed potatoes and peas on a cold autumnal evening, drinking the same beer that went into the stew.
Heat the fat in a flameproof casserole dish until very hot, and brown the meat on all sides. I use oil, for reasons of kashrut, but butter is traditional. However, if you do use butter, add a splash of oil because it will help to prevent it from burning. Try not to turn the meat too much whilst it is browning — allow about three minutes per side — and don't worry if it sticks a little, it'll add to the meaty-flavour. When it's done, remove the meat to a plate.
Reduce the flame beneath the casserole to medium and add a drop more fat. Tip in the onions and fry them until they're beginning to brown, too. This might take about fifteen minutes with the lid on. You don't need to pay them too much attention, just stir occasionally.
When the onions are ready, add a spoonful of flour and stir well.
Return the meat and any juices that have accumulated whilst it has been resting to the pan, along with 250ml (8floz) of beer, the bay leaves, thyme, and salt and pepper. Mix it thoroughly and scrape the bottom of the pan well to release the sticky, meaty bits that caught during the browning process.
Bring it all to a fast simmer and then reduce the flame to the slowest possible. Put the lid on the pan and leave it to cook very gently for between two and three hours. Check on it periodically to ensure it doesn't require any more liquid, if so, add some more beer.
It's almost ready when the meat is tender enough to break up under the pressure of a fork and is coated in a thick, brown sauce. What remains is to stir in the mustard and brown sugar, check the flavour balance, and allow everything to cook through for five minutes. Then you're ready to serve it. Traditionally, Belgians would opt for pommes frites or boiled potatoes as an accompaniment, but our option of mash worked wonderfully. Have whatever you prefer, and enjoy.