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 two 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 1/2 teaspoons salt. Serve hot, with sour cream, pepper, and pumpkin seeds, if desired.