Melt butter in a large pot over medium-high heat. Add onion and cook until onion is translucent; 4-6 minutes.
Add squash, apple and water. Simmer uncovered until the squash and apple are fork-tender, about 30 minutes.
Stir in brown sugar, honey, cinnamon and allspice. Simmer for 2-3 more minutes.
Take off heat and cool for about 10 minutes.
Working in batches, puree soup in blender until smooth. Pour pureed soup back in pot and heat over medium-heat. Season with salt and pepper. Once hot, lower heat to a simmer until ready to serve.
Spoon into bowls, Drizzle with creme fraiche and sprinkle with toasted hazelnuts.
Recipe by Farmhouse 1820 at https://farmhouse1820.com/butternut-squash-apple-soup/