Press Sauté; melt butter in Instant Pot®. Add onion; cook and stir 2 minutes. Add squash, coconut milk, 1 tablespoon brown sugar, salt, cinnamon, nutmeg and allspice; mix well.
Secure lid and move pressure release valve to Sealing position. Press Manual or Pressure Cook; cook at high pressure 6 minutes.
When cooking is complete, press Cancel and use quick release.
Stir ginger into squash mixture. Use immersion blender to blend squash until smooth. (Or process in food processor or blender.) Stir in lemon juice. Sprinkle individual servings with remaining 1 tablespoon brown sugar, if desired.