Preheat the oven to 400 F. Toss grapes and squash cubes together in a large bowl with the oil and salt and pepper, to taste. Spread them in a single layer on a baking sheet and roast for about 20 to 30 minutes, until the grapes have begun to slightly shrivel and the squash is tender. Set aside.

Heat a large nonstick skillet over medium-high heat and cook the pancetta until it begins to brown slightly. Add the celery and onions and cook until they are tender and the onions are translucent, about 5 to 6 minutes. Add the garlic and cook for 1 minute more. Remove from the heat.

In a large bowl, combine the cornbread, pancetta mixture, grapes, squash, walnuts, sage, and salt and pepper to taste, if desired. Set aside.

In a medium bowl, whisk together the broth, butter, vinegar, and egg. Pour over the cornbread mixture and toss well to combine.

Spray a 9×13-inch baking dish with cooking spray. Spoon the dressing evenly into the dish. (At this point, you can either bake immediately or cover and refrigerate overnight to bake the next day.)

Bake, covered with foil, at 350 F for 20 minutes. Uncover and bake another 20 minutes, or until warmed through and golden brown.