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1
Place a wide pan over medium-low heat. Melt the butter with the oil, then add the sliced onions. Cook slowly for 30–40 minutes, stirring occasionally, until they are deeply and darkly caramelised — nearly jammy in consistency. The onions are ready when they have collapsed entirely and taken on a rich, mahogany colour throughout.
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2
Heat the oven to 160°C. Season the beef slices. Working in batches, brown them deeply in a hot pan with a little oil, ensuring a dark crust forms on each side. Remove and set aside. The surface of each slice should be deeply coloured — not grey — before it leaves the pan.
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3
In a deep casserole, layer the browned beef and caramelised onions alternately, scattering fresh thyme from the bouquet garni as you build. Pour over the dark Belgian ale and the beef stock. Add the dark brown sugar, the red wine vinegar, and the bouquet garni. Bring the contents to a simmer over direct heat. The liquid should be visibly moving but not boiling hard.
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4
This is the step on which the dish lives or dies. Spread the country bread slices generously with Dijon mustard and lay them mustard-side down directly on top of the simmering stew, covering the surface. As the braise proceeds, this bread lid dissolves into the sauce, thickening it naturally while the mustard permeates throughout — a result no roux can replicate. If the bread is omitted or under-mustarded, the sauce will be thin and the characteristic sharpness absent.
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5
Cover the casserole and transfer to the oven at 160°C. Braise for 2.5–3 hours. The carbonnade is done when the beef slices yield completely to the pressure of a spoon and the sauce has turned a dark mahogany colour, thick and glossy, carrying the malt of the beer, the bitterness of the onions and sugar, and the sharp heat of the mustard in balance.