To make the soup, peel the butternut squash with a vegetable peeler.
Slice the squash in half and remove the seeds with a melon baller or spoon.
Dice the squash into chunks.
In a large pot or Dutch oven, melt the butter and add the onion and garlic. Sauté until tender, about 10 minutes.
Add the squash, broth, and herbs.
Bring the soup to a boil. Reduce the heat to a simmer and cover the pot. Cook until the squash is very tender, about 20 minutes. Purée the soup in a blender in small batches, or use an immersion blender. Add the cream and sugar if desired. Taste for seasoning and add salt and pepper as needed.
To make the croutons, slice the baguette diagonally and butter each side. Broil until golden, about 1 minute. Flip the bread over and top with the cheese, herbs, and salt and pepper. Broil until the cheese is melted and bubbly. Serve alongside or on top of the soup.