If you are starting with a whole stalk of lemongrass, remove the top woody part. Cut the softer, bottom piece in to two 2-3 inch pieces. Smash the side of the stalk with a knife blade to bruise. Set aside.
In a slow cooker, combine the cubed squash, coconut milk, chicken stock, curry paste, ginger, fish sauce and brown sugar. Cover and cook on low for 3 1/2 - 4 hours, or until squash is tender. Discard the ginger and the two lemongrass pieces.
Using a whisk (or potato masher, or immersion blender), mash and stir squash to make a smooth soup. Stir in shrimp and cook a few minutes, until they are warmed. (If using raw shrimp, cover soup and cook on high for about 15 minutes, to cook shrimp).
Taste soup and add salt and pepper, to taste. (Salt especially is needed to really bring out the flavours!)
Spoon in to serving bowls and garnish with fresh mint or basil.