In pot, melt butter over medium heat; add onions, thyme and 1/4 teaspoons each salt and pepper and sauté for 2 minutes or until starting to soften. Reduce heat to medium-low, cover and cook, stirring often, for about 20 minutes or until onions are very soft. Uncover and cook, stirring often, for 15 to 20 minutes or until caramelized. Increase heat to medium-high; add garlic and sauté for 1 minute. Add sherry and bring to a boil, scraping up bits stuck to pot. Add beef stock and water and bring to a boil. Reduce heat and boil gently for about 10 minutes or until flavours are blended.
Preheat broiler. On a baking sheet, lightly toast both sides of baguette slices. Spread one side with softened blue cheese.
To serve, stir cream into hot soup and season to taste with salt and pepper. Ladle soup into ovenproof soup bowls . Float one crouton in each bowl and broil for about 2 minutes, or until cheese is melted. Serve immediately.