Pat the beef cubes dry with paper towels to ensure a good sear. Season generously with salt and pepper, then dredge in the flour, shaking off any excess. This step helps create a thick, flavourful base for the stew.

In a heavy bottomed Dutch oven or large oven safe pot, heat 1 tablespoon of butter and 1 tablespoon of oil over medium high heat. Brown the beef cubes in batches to avoid overcrowding, ensuring each piece develops a deep golden crust. Transfer browned beef to a plate and set aside.

Lower the heat to medium. Add the remaining butter and oil to the pot. Sauté the onions until they become soft and caramelized, about 10-15 minutes. Stir occasionally to prevent burning and scrape up any brown bits from the bottom of the pot for extra flavour.

Add the minced garlic and cook for another minute until fragrant. Deglaze the pot with the beer, scraping up any stuck bits. Allow the beer to simmer for 3-4 minutes to burn off the alcohol slightly and meld the flavours.

Return the browned beef to the pot. Add the beef stock, mustard, vinegar, brown sugar, thyme, and bay leaves. Stir to combine, ensuring the liquid covers most of the beef. Bring to a gentle simmer.

Preheat your oven to 325°F (160°C). Cover the pot with a tight fitting lid and transfer it to the oven. Allow the stew to braise for 2 ½ to 3 hours, stirring halfway through, until the beef is tender and the sauce has thickened.

Just before the stew is finished, spread a thin layer of Dijon mustard on the toasted bread slices. Lay them on top of the stew in the pot for the last 30 minutes of cooking. This step enriches the stew with additional flavour and helps thicken the sauce further.

Remove the bay leaves and thyme sprigs before serving. Garnish with freshly chopped parsley. Serve the Carbonade Flamande hot alongside crispy Belgian fries or buttered egg noodles, and pair with a chilled glass of Belgian beer for an authentic dining experience.