Melt 2 Tbs. butter in a heavy large pot over medium-high heat. Add yellow onions and cook until very soft and carmelized, stirring frequently, about 40 minutes. Add wine and sherry and bring to a boil, scraping any browned bits from the bottom of the pan. Add stock, Worster, Thyme, oregano, pepper and bay leaf. Reduce heat and simmer 30 minutes, stirring occasionally.
Melt remaining 1 Tbs. butter in a heavy skillet over medium heat. Add garlic, shallots and green onions and cook until golden, stirring frequently, about 10 minutes. Add to soup and stir to combine. (Can be prepared ahead and refrigerated, bring to a simmer before continuing.)
Preheat broiler. Ladle soup into 4 broilerproof soup crocks. Top each with 2 toasted bread slices and 2 cheese slices. Broil until cheese bubbles.