For Lamb Marinade: In saucepan combine beer, honey, thyme and peppercorns. Cook over medium-low heat, stirring until honey dissolves; chill. Reserve 4-1/2 cups marinade for cipollini onions and reserve 4-1/2 cups marinade for sauce. Place lamb in non-reactive pan and pour remaining chilled marinade over lamb. Cover, refrigerate and marinate for 2 to 4 hours. Remove lamb from marinade and discard marinade.
To cook lamb: In large skillet, heat 1 tablespoon oil over medium-high heat. Quickly brown lamb on both sides in batches, adding additional oil as needed. Place loins in shallow roasting pan. Roast in 400°F oven for 16 to 18 minutes or to desired degree of doneness: 145°F medium-rare, 160°F medium, or 170°F for well. Cover and let stand 5 to 10 minutes. Internal temperature will rise approximately 5 to 10 degrees upon standing.
To make sauce: Pour reserved marinade for sauce and juice from cooked onions into a saucepot and cook over medium-high heat until reduced to about 3-3/4 cups. Add the lamb broth from demi glace and heat through. Add butter and stir until melted. Season with salt and pepper, keep warm.
For Chipollini Onions: In large skillet, heat 2 tablespoons oil over medium-high heat. Add onions, cook and stir until browned on all sides. Cook in batches adding oil as needed. Using slotted spoon transfer onions to roasting pan. Pour off excess oil and deglaze the skillet with reserved Guinness® marinade. Cook until liquid is reduced to 3 cups. Add chicken broth and bring to a boil; pour over onions. Cover and bake in 400°F oven for 30 minutes or until onions are tender. Remove onions from juice and keep warm. Reserve juice for lamb sauce. Just before serving, melt butter and honey in large skillet, add onions and cook over medium-high heat until glazed; season with salt and pepper.
For Roasted Yukon Gold Potatoes: Cut potatoes into neat 3/4-inch squares. Melt 4-1/2 tablespoons butter in large skillet; add half of the potatoes and cook over medium-high heat, tossing gently until lightly browned on all sides. Repeat process preparing all potatoes. Spread on full sheet pan and season with salt and pepper. Bake in 400°F oven until tender, about 10 minutes. Keep warm and toss with parsley just before serving.
To serve: Divide potatoes among 12 warm plates. Carve lamb loin and fan over potatoes; top with onions. Drizzle sauce all around the plate and garnish with chervil sprigs. Serve with remaining sauce.