1. Caramelize onions with olive oil over medium low heat until soft and brown, about 20 minutes. Add chicken broth, thyme, salt and pepper. Bring broth to a boil, then cover and reduce heat to a simmer, cook slowly for an additional 30 minutes.
2. Preheat the oven to 400°F. Blend together the blue cheese and butter, set aside. Arrange bread on a baking sheet and toast for 3 to 4 minutes on each side, until golden brown. Remove and cover one side of the bread with the cheese mixture. Return to the oven until cheese melts evenly over the bread, about 3 minutes.
3. To serve, ladle the soup into bowls and top with two croutons.