Melt butter and olive oil in a large soup pot over medium
heat. When the butter has melted, add onion and cook, stirring
occasionally, until onion is completely softened, about
4 to 5 minutes. Stir in the pumpkin and cook another 3 minutes.
Season well with salt and pepper. Stir in the chicken broth
and bring the soup to a simmer, stirring occasionally. Reduce
heat to low and cook 10 minutes. Add brown sugar, crushing
it first in your hands to smooth out the lumps. Mix well.
Meanwhile, make the Herbed Goat Cheese Mousse: Using an
electric or stand mixer, whip the cream until it holds together
in soft peaks. With the machine off, crumble in the goat
cheese, salt, pepper, and thyme. Turn machine on to medium
speed and whip goat cheese and cream together until uniform,
about 1 minute, scraping sides of bowl as necessary. Chill
until ready to use. The soup and the goat cheese mousse
can be made several hours ahead of time. Reheat soup and
serve hot, topping each bowl with a tablespoon-sized dollop
of Herbed Goat Cheese Mousse. Serves 4 to 6.