Set a rack in the upper third of the oven, then preheat to broil.
Cover a baking sheet with aluminum paper, then add the four grapefruit halves, cut side up.
In a small bowl, combine the brown sugar, butter and nutmeg until the ingredients are moistened. Press a spoonful of brown sugar mixture onto the surface of each grapefruit. Broil 3 to 5 minutes, or until the sugar is melted and charred in spots. Remove from the oven and let cool.
In a large bowl, whisk the whipping cream until soft peaks form. Whisk in the maple syrup.
Serve the broiled grapefruits generously topped with maple cream.