Melt butter in a large saucepan over medium heat. Cook onion until fragrant, about 2 minutes. Add ginger, garlic, and squash; cook, stirring occasionally, until fragrant, 6 to 8 minutes. Stir in 4 cups water. Bring to a boil; reduce heat. Simmer until squash is tender, 20 minutes.
Puree soup in 2 batches. When blending hot foods, allow the heat to escape to prevent splattering. Remove the cap from the hole of the blender's lid, and cover with a dish towel.
Stir in juice and 1 ½ teaspoons salt. Serve hot, with sour cream, pepper, and pumpkin seeds, if desired.
