Farro and Broccoli Rabe Gratin

 Heat 1 tablespoon oil in large saucepan over medium heat until shimmering. Add onion and salt and cook until softened and lightly browned, 5 to 7 minutes. Stir in farro and cook, stirring occasionally, until lightly toasted, about 2 minutes. Stir in 2½ cups water, broth, and miso; bring to simmer; and cook, stirring often, until farro is just tender and remaining liquid has thickened into creamy sauce, 25 to 35 minutes.

 Meanwhile, toss panko with 1 tablespoon oil in bowl and microwave, stirring occasionally, until golden brown, 1 to 2 minutes. Stir in Parmesan and set aside.

 Bring 4 quarts water to boil in Dutch oven. Add broccoli rabe and 1 tablespoon salt and cook until just tender, about 2 minutes. Drain broccoli rabe and set aside. Combine remaining 1 tablespoon oil, garlic, and pepper flakes in now-empty pot and cook over medium heat until fragrant and sizzling, 1 to 2 minutes. Stir in reserved broccoli rabe and cook until hot and well coated, about 2 minutes. Off heat, stir in beans, tomatoes, and farro mixture. Season with salt and pepper to taste.

 Adjust oven rack 10 inches from broiler element and heat broiler. Transfer bean-farro mixture to broiler-safe 3-quart gratin dish (or broiler-safe 13 by 9-inch baking dish) and sprinkle with reserved panko mixture. Broil until lightly browned and hot, 1 to 2 minutes. Serve.