Heat 1 tablespoon oil in a medium skillet over medium-high heat. Cook onions and remaining tablespoon rosemary for 2 minutes. Reduce heat to medium, and cook until onions are golden brown, about 2 minutes more. Add 1 tablespoon oil. Flip onions, and season with a pinch of salt. Cook, flipping, until onions are tender and browned on both sides, 8 to 10 minutes more. Remove from heat. Stir in vinegar and remaining 2 tablespoons oil. Pour mixture over farro; toss. Season with 1 teaspoon salt and some pepper. Stir in red grapes. Let stand for 20 minutes.
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