8 tablespoons (1 stick) chilled unsalted butter, divided
1 large shallot, minced
1/2 cup balsamic vinegar
2 large fresh rosemary sprigs
2 pounds mixed sweet onions, large shallots, baby leeks and green onions
2 tablespoons olive oil
4 1½ inch thick New York strip steaks (each about 12 ounces)
2 tablespoons orange juice
Melt 6 tablespoons butter in heavy small saucepan over medium heat.
Add minced shallot and cook until translucent, about 2 minutes.
Add vinegar and rosemary then simmer until syrupy and reduced to 1/2 cup, about 6 minutes. Remove from heat (balsamic sauce will separate as it stands).
Prepare barbecue grill (medium-high heat). Peel sweet onions, then cut into 1/2 inch thick rounds. Skewer the meat horizontally with toothpicks to keep intact. Peel shallots; cut in half lengthwise. Trim root ends and tops of leeks. Cut in half lengthwise; rinse to remove any dirt. Trim root ends of green onions. Place sweet onions, shallots, leeks and green onions on rimmed baking sheet and drizzle with oil and sprinkle with salt and pepper. Sprinkle steaks with salt and pepper. Grill sweet onions, shallots, leeks and green onions until tender, turning often, about 5 minutes for green onions and 10 minutes for sweet onions, shallots and leeks. Move onions to cooler part of grill to keep warm. Grill steaks to desired doneness, about 7 minutes per side for medium rare. Transfer steaks and onions to a platter; cover. Remove rosemary sprigs from balsamic juice and bring to simmer. Remove saucepan from heat; add remaining 2 tablespoons chilled butter and whisk until melted and sauce is smooth. Season the sauce to taste with salt and pepper. Drizzle some of sauce over steaks and onions. Serve passing remaining sauce separately.