Preheat broiler or prepare a charcoal fire and let burn to a gray ash. Stir orange juice concentrate with soy sauce, mustard and butter in a small saucepan over medium-high heat until smooth, 2 minutes.
Thread beef onto 8 long metal skewers, dividing evenly and alternating with mushrooms, onion pieces and cherry tomatoes. Brush orange mixture evenly over threaded skewers and season with salt and pepper. Bring any remaining glaze to a boil in saucepan, then remove from heat and reserve.
Set broiling pan or grill about 6 inches from heat source. Broil or grill skewers for 6 to 8 minutes, turning often, until meat and mushrooms are browned and onions are just charred on edges. Serve kebabs warm with remaining glaze on the side.