In large shallow roasting pan, combine onions, shallots, garlic and oil. Roast, stirring every 5 minutes, 20 to 30 minutes or until onions are golden. Remove from oven, mash garlic halves and pour in 1 cup of the broth; stir, scraping bottom of pan to loosen and dissolve any caramelized bits. (The liquid will become quite dark.)