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Bring balsamic vinegar to a boil in a small, heavy saucepan. Reduce heat to medium; cook until reduced to 1/4 cup (about 5 minutes). Remove from heat.
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Heat oil in a large nonstick skillet over medium-high heat. Add onion, and sauté 10 minutes or until tender. Sprinkle with 1/4 teaspoon salt; sauté 18 minutes or until onions are golden brown. Remove from heat.
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Preheat broiler.
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Sprinkle steak with remaining 1/4 teaspoon salt, pepper, and thyme. Place steak on a broiler pan coated with cooking spray; broil 6 minutes on each side or until desired degree of doneness. Cutsteak diagonally across grain into thin slices. Serve steak over onions; drizzle with balsamic glaze.