Heat 1 tablespoon of butter and the vegetable oil in a large Dutch oven or heavy bottomed pot over medium to high heat. Season the beef cubes generously with salt and black pepper. Brown the beef in batches to avoid overcrowding, ensuring each piece develops a deep, caramelized crust. Remove and set aside.

Reduce the heat to medium, add the remaining butter, and sauté the sliced onions until golden and soft, about 10 minutes. Stir occasionally, allowing them to pick up the caramelized bits from the pan.

Sprinkle the flour over the onions and stir continuously for 1-2 minutes. This step helps thicken the stew later. Gradually pour in the beer, stirring constantly to prevent lumps from forming.

Return the browned beef to the pot, along with the beef stock, apple cider vinegar, Dijon mustard, and brown sugar. Stir well to combine. Bring the mixture to a gentle simmer.

Tie the bay leaves and thyme together with kitchen twine (or use a small spice bag) and add them to the pot. This will allow easy removal later while infusing the dish with their aroma.

Spread the slice of country bread with Dijon mustard and place it on top of the stew, letting it dissolve into the sauce as it simmers. This traditional step adds richness and helps thicken the sauce.

Cover the pot and reduce the heat to low. Let the stew gently simmer for at least 2.5 hours, stirring occasionally. If the sauce thickens too much, add a splash of water or stock to maintain a smooth consistency.

For added complexity, stir in the cocoa powder during the last 30 minutes of cooking. It deepens the flavour without making the dish taste chocolatey.

Check the beef for tenderness, it should be fork tender and almost falling apart. Remove the bay leaves and thyme stems. Adjust seasoning with additional salt, pepper, or a dash more vinegar if desired.

Serve Stoofvlees hot with traditional Belgian fries (frites), crusty bread, or mashed potatoes. Garnish with fresh thyme if desired. Pair with a glass of the same Belgian beer used in the stew for an authentic experience.