Step 1

Place the farro into a fine-mesh strainer and rinse it under cold running water. In a medium saucepan over high heat, bring the broth to a boil. Add the bay leaf and stir in the farro, then return to a boil. Reduce the heat to medium-low, cover and simmer, until the farro is tender, about 30 minutes. (The cooking time will vary depending on the type of farro.) Drain off any excess liquid and discard the bay leaf. (The strained broth can be saved for another use, such as a soup or stew.)

Step 2

In a large high-sided skillet over medium heat, heat the oil until shimmering. Add the onions, and cook, stirring frequently, until softened, about 3 minutes. Season with the salt, reduce the heat to medium-low and continue to cook, stirring frequently, until the onions are pale golden, about 25 minutes. Add the garlic and cook until tender and the onions are a deeper golden color, about 10 minutes. As the onions cook, add a splash of broth or water if the pan seems dry. Add the thyme and black pepper, and then stir in the vinegar, scraping up any brown bits on the bottom of the pan.

Step 3

Position one rack in the middle of the oven and another about 6 inches away from the broiling element, and preheat to 375 degrees. Brush a 2-quart baking dish, such as an 8-inch-square baking dish, with olive oil.

Step 4

Stir the cooked farro and 1/2 cup of the cheese into the skillet with the onion mixture. (If you have prepared them in advance, combine them in a large bowl and let them come to room temperature.) Transfer the mixture to the baking dish and sprinkle the remaining 3/4 cup of cheese on top. Cover with foil and bake for about 20 minutes or until warmed through and the cheese is melted. Remove the foil and turn on the broiler. Transfer the baking dish to the top rack and broil for 2 to 3 minutes, or until the cheese is nicely browned. Garnish with the parsley and serve hot.