Melt 1/4 cup butter in heavy large dutch oven over medium-high heat. Add shallots, all onions and leek; saute until golden, about 15 min. Add 1 tbsp garlic and cloves; saute 1 min. Sir in broth and Sherry. Bring to boil. Reduce heat to medium-low, cover and simmer 30 min. (can be prepared a day ahead. Cover and refrigerate. Bring to simmer before continuing.)
Preheat broiler. Combine 2 tbsp. melted butter and 1 tsp. garlic in small bowl. Place bread on baking sheet and brush with half of garlic butter. Broil bread until half of garlic butter. Broil bread until lightly toasted, about 2 min. Turn bread over and brush with remaining garlic butter. Broil until lightly toasted, about 2 minutes. Sprinkle bread with parmesan cheese.
Preheat oven to 500. Ladle soup into bowls. Top each bowl with 1 crouton. Place 1 slice of swiss cheese and 1 slice of mozzarella cheese atop each crouton. Bake until soup is hot and cheese melts, about 10 min.
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