Preheat the oven to 375°F. Grease a 9 x 13-inch baking dish. Heat 1 tablespoon of the oil in a large skillet over medium-high heat. Add half the squash and season liberally with salt and pepper. Cook stirring occasionally, until browned, about 5 to 7 minutes, then transfer to the baking dish. Repeat browning of remaining squash with another tablespoon of olive oil and more salt and pepper. Transfer to baking dish. Heat another tablespoon of the oil in the skillet and add the onion and rosemary. Season with salt and pepper and cook until softened, about 5 minutes. Pour in the broth and scrape up any browned bits on the bottom. Bring to a simmer and pour into baking dish. Cover with foil and bake until squash is very tender, about 40 minutes. Raise the oven temperature to 450°F. Remove the dish from the oven and take off the foil. Sprinkle the grapes over the top. In a medium bowl, mix together the breadcrumbs, Parmesan and remaining 3 tablespoons olive oil and season with salt and pepper. Sprinkle over the squash. Bake until top is browned and bubbling, or about 10 to 15 minutes longer.