Begin by seasoning the rabbit pieces with salt and black pepper. Lightly coat them with Dijon mustard, ensuring an even layer. This will enhance the flavour while helping to create a beautiful caramelization during searing.

Heat the butter and vegetable oil in a large, heavy bottomed Dutch oven over medium high heat. Once hot, sear the rabbit pieces until they develop a golden brown crust on all sides. Remove from the pan and set aside.

Lower the heat to medium and add the diced bacon. Cook until crisp and rendered, then add the onions. Stir occasionally until they become soft and lightly caramelized, about 10 minutes. Add the minced garlic and cook for another minute until fragrant.

Sprinkle the flour over the onion bacon mixture and stir well to coat. Cook for 1-2 minutes to remove the raw flour taste, then deglaze the pan with red wine vinegar, scraping up any browned bits from the bottom.

Pour in the Gueuze beer gradually, stirring continuously to incorporate it smoothly into the sauce. Add the bouquet garni, brown sugar, and stock, then return the rabbit pieces to the pot. Ensure the rabbit is partially submerged in the liquid.

Reduce the heat to low, cover, and let the rabbit simmer gently for about 1.5 hours, turning the pieces occasionally. The meat should become tender and almost fall off the bone.

After an hour of cooking, add the raisins if using. Taste the sauce and adjust the seasoning with more salt, pepper, or a bit more sugar if needed.

Once the rabbit is fully tender, remove it from the sauce and keep warm. Discard the bouquet garni, then increase the heat slightly to allow the sauce to reduce and thicken for about 10 minutes, stirring occasionally.

Return the rabbit to the pot, ensuring it is well coated with the thickened sauce. Serve hot, garnished with freshly chopped parsley. Traditionally, Lapin à la Gueuze pairs beautifully with stoemp (Belgian mashed potatoes with vegetables) or crusty bread to soak up the sauce.