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. 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.