In a large saucepan over medium heat, melt butter. Add onion and cook until fragrant, about 2 minutes. Add ginger, garlic, squash and curry powder and cook, stirring occasionally, until fragrant, 6 to 8 minutes.
Stir in 4 cups water. Bring to a boil and then reduce heat. Simmer until squash is tender, 20 minutes.
In a blender or in the saucepan with a hand blender, puree soup (if doing in a blender, you will need to do it in two batches). Note that you should remove the cap from the hole of the blender's lid and cover with a dish towel to allow heat to escape.
Once soup is pureed, stir in orange juice and 1 ½ teaspoons salt. If you want soup thinner, add more orange juice. Serve hot and top with sour cream, pepper, and pumpkin seeds, if desired.