Preheat grill to high heat. Lightly coat onions with cooking spray. Place onions on grill rack; grill 10 minutes on each side or until tender. Place onions in a medium bowl; cover tightly with foil. Keep warm. Lightly coat steak with cooking spray; sprinkle steak with 3/4 teaspoon kosher salt and 3/4 teaspoon black pepper. Place steak on grill rack; grill 6 minutes on each side or until desired degree of doneness. Let stand 3 minutes. Cut steak diagonally across grain into thin slices. Add tomatoes, balsamic vinegar, 1/4 teaspoon kosher salt, and 1/4 teaspoon black pepper to onions; toss gently to combine. Divide steak evenly among 6 plates; top with 1/2 cup tomato mixture. Cut avocado wedges in thirds crosswise. Top each serving with 4 avocado pieces.
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