In a large sauté pan over medium heat, add the butter, stirring until the butter has melted and the foam has subsided. Add the onions and leeks; season lightly with salt, stirring occasionally for 10 – 12 minutes or until brown and caramelized. Deglaze the pan with Achel Bruin (leaving the yeast in the bottle) and reduce by half. Remove from the heat and place in a metal bowl to cool. Once the onion | leek mixture is cool, add the ground lamb and season with salt and pepper; mix to distribute equally and make 4 burger patties. Set patties on a plate and let sit as your charcoal or gas grill heats up.