Blanch onions in a large pot of boiling water 1 minute and drain, then cool under cold running water. Trim root ends and peel. Cook onions in butter in a 10-inch heavy skillet over moderate heat, stirring, 3 minutes (onions will not brown).
Add salt and regular-grind pepper to taste and enough water to just cover onions and boil, partially covered, stirring occasionally, until onions are tender but not falling apart, about 15 minutes.
Transfer onions to a bowl with a slotted spoon and if necessary boil juices until reduced to about 1/4 cup. Toss onions with juices.
Combine coarsely ground pepper, chopped rosemary, garlic, and salt, then rub into both sides of steak.
Cut meat into 3/4-inch cubes.
Put 1 onion, then 1 steak cube (thread through unpeppered sides) on each rosemary branch or skewer to make 60 kebabs.
Preheat broiler.
Arrange some of kebabs in a row, a peppered side facing up, along 1 long side of oiled rack of a broiler pan so that leaf ends of rosemary branches and blunt ends of skewers point toward middle of pan. Cover exposed skewer or branch ends with a sheet of foil (don’t cover beef or onions). Arrange another row of kebabs over foil, covering exposed ends in same manner. Continue adding rows of kebabs and layers of foil until pan is full, making sure exposed ends of last row of branches or skewers are covered with foil. (To broil kebabs in batches, keep remaining kebabs on a tray, covered with plastic wrap and chilled.)
Broil kebabs 2 inches from heat until beef is seared on 1 side but still medium-rare, 2 to 3 minutes.
Serve immediately.
·Onions may be braised 2 days ahead and chilled, with juices, in a sealed bag.·You can chill raw kebabs, covered, on broiler pan or a tray 4 hours.·Although all kebabs will fit on 1 broiler pan, you may prefer to broil kebabs in batches so they are not all ready at once.
