In large nonstick pot, heat olive oil and sauté onions and portabellas on medium heat, stirring, until onions are golden and very soft, about 10 to 15 minutes. Add sugar and garlic; continue cooking 15 to 20 minutes more, stirring. Add sherry, stirring to loosen any brown bits from pot.
Add mustard, thyme, and flour, and stir 1 minute. Add broth and wine. Bring to boil, reduce heat, and cook 20 to 30 minutes longer. Season to taste.
In bowl, ladle hot soup, top with bread and sprinkle with mozzarella and Parmesan cheese. Microwave or broil (in oven-proof bowl) until cheese melts. Makes about 8 cups.
Spicy Advice: The baby portabellas give the soup a rich earthy flavor.