Preheat a grill to medium high. Combine the steak, red onion and zucchini in a large bowl. Add the barbecue sauce, chili powder, Worcestershire sauce, 1/2 teaspoon salt, and pepper to taste; toss to coat. Let stand 5 minutes. Meanwhile, mash the butter with the parsley, lemon zest and a pinch of salt in a small bowl; set aside. Transfer the steak to the grill and cook 4 to 5 minutes per side for medium rare; remove to a cutting board and let rest. Add the vegetables to the grill and cook, turning occasionally, until crisp-tender and charred in spots, about 8 minutes. Cut the steak into 4 pieces. Top each piece with some of the lemon-herb butter. Serve with the grilled vegetables.