Melt the butter in a large saucepan over medium heat. Add the onions and sprinkle with salt and pepper. Sauté until the onions are translucent, about 4 minutes. Stir in the curry powder and cook for another minute.
Add the broth, pumpkin puree, and red pepper flakes. Bring to a simmer and cook for 5 minutes.
Transfer to a blender and puree until smooth. Depending on the size of your blender, you may need to work in batches. (Alternatively, you can use an immersion blender to puree the soup right in the pot.) Return the soup to the pot and stir in the heavy whipping cream. Adjust the seasonings to taste.
Garnish with dollops of creme fraiche and pumpkin seeds.
Dairy Free Option: Use avocado oil in place of the butter and full-fat coconut milk in place of the heavy whipping cream. Omit the creme fraiche garnish.