1. Preheat oven to 250 degrees. Arrange red grapes in a single layer on a rimmed baking sheet. Sprinkle with 3/4 teaspoon salt. Place Concord grape bunches in the center of red grapes. Bake until grapes have shrunk to about half their size but are still juicy, about 1 hour 30 minutes. Let cool.

2. Meanwhile, combine farro, 1 tablespoon rosemary, and 1 1/2 teaspoons salt in a medium saucepan; cover with water by 1 inch. Bring to a simmer, and cook until tender, about 25 minutes. Drain, and transfer to a bowl.

3. 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.

4. Gently stir in greens just before serving. Arrange salad on a platter, and garnish with bunches of Concord grapes.