When he was younger, my son was stationed in Bamberg, Germany, with the US Army. He naturally partook of many a German dish, and this was one which is very popular in the Bamberg area, as a main course.
Recipe: The New German Cookbook
For the onions: Peel the onions very carefully, leaving the tops and root ends intact; set aside.
In a large heavy saucepan, over moderate heat, bring the broth to a boil. Add the onions, cover, and parboil for 5 minutes. Lift the onions from the broth and cool until easy to handle. Reserve the broth.
Slice 1 inch off the top of each onion and save for another use. Using a small sharp paring knife, make crisscross cuts deep into the center of each onion, taking care not to nick the outer two layers. With a melon-baller, very carefully scoop out the centers of each onion, leaving walls two-ply thick.
Chop the onion centers moderately fine, measure out and reserve 1-1/2 cups for the filling, and save the balance for another recipe. Let the hollowed-out onions drain upside down on paper towels while you prepare the stuffing.
For the Stuffing: Crumble the Kaiser roll into the water in a small bowl and let stand 10 minutes. Squeeze the pieces of roll lightly to extract some of the water, then place in a food processor fitted with the metal chopping blade. Add remaining stuffing ingredients and pulse 8 to 10 times to combine.
Spoon the stuffing into the scooped-out onions, mounding it up a bit on top. Stand the onions in an ungreased round casserole, just large enough to accommodate them without their touching one another. Prop the onions, as needed, with crumpled aluminum foil so they stand straight without wobbling. Pour the reserved broth into the casserole to a depth of 1 inch.
Cover the onions and bake in a preheated 400-degree oven for 1 hour, just until they are tender and the stuffing is cooked through. Using a slotted spoon, lift the onions to a heated round platter, cover loosely and keep warm.
For the Sauce: Pour the casserole juices into a small heavy saucepan, add the beer, and boil uncovered for 5 minutes over high heat until the liquid is reduced by about half. Add the butter and whisk until it melts. Carefully pour the broth mixture around the onions on the platter, covering the well completely.