Remove skin from garlic without peeling or separating the cloves. Wrap it in foil. Bake at 350° for 1 hour. Let cool 10 minutes. Separate cloves, squeezing to extract the inside. Reserve half for another use.
In a large pot on medium high heat, saute the pancetta 5 minutes or until crisp. Remove pancetta and set aside. Add onion and 1 tablespoon oil to drippings in pan and sauté 5 minutes. Reduce heat to medium-low; cook 25 minutes to brown onion, stirring frequently until tender. Add vinegar.
Raise oven to 375°.
Add garlic pulp, pancetta, squash, broth, thyme, salt, pepper, cannellini, and bay leaf to onions, stirring well.
Pulse bread in a food processor about 10 times to make 1 cup dry crumbs. Combine crumbs, Parmesan cheese, and 1/2 teaspoon olive oil. Sprinkle crumbs over squash mixture.
Cover and bake for 50 minutes or until squash is tender. Uncover and bake an additional 15 minutes or until topping is browned. Discard bay leaf. Sprinkle with parsley.
You can roast the garlic, carmelize the onions, and assemble the casserole all in advance if you want.