Whisk together the stout, 1/4 cup water, oil, soy sauce, fennel, sugar, thyme and garlic in a baking dish. Toss the lamb in the marinade to coat, and then cover and refrigerate at least 1 hour and up to 4 hours.
Preheat a grill or grill pan. Sprinkle the lamb with salt and pepper, and then thread 2 or 3 pieces onto each skewer so that the meat lays flat.
Grill the lamb until slightly charred and medium-rare, 2 minutes per side. Serve on a platter with the Honey Dipping Sauce on the side.
Honey Dipping Sauce:
Whisk together with the honey, mustards, horseradish, mint and fennel seeds. Season with salt and pepper, and then cover and refrigerate at least 30 minutes to allow the flavors to meld.