Melt butter in large pot over medium heat. Add onion and garlic and sauté until tender, about 10 minutes. Add broth, all squash and herbs; bring to boil. Reduce heat, cover and simmer until squash is very tender, about 20 minutes.
Working in batches, puree soup in blender. Return soup to same pot. Stir in cream and sugar; bring to simmer. Season with salt and pepper. (Can be made 1 day ahead. Chill. Rewarm over medium heat before serving.).
Preheat broiler. Butter 1 side of each bread slice. Arrange bread, buttered side up, on baking sheet. Broil until golden, about 1 minute. Turn over. Sprinkle cheese, then thyme and sage over. Sprinkle with salt and pepper. Broil until cheese melts, about 1 minute. Ladle soup into bowls. Top each with croutons and serve.