Purée the Bananas: In a blender or food processor, blend the ripe bananas until completely smooth and lump-free.
Whip the Cream: Using a large, chilled mixing bowl, beat the heavy cream on medium-high speed until stiff peaks form.
Add Sweetener: Gently whip in the powdered sugar until just combined and soft peaks form. Be careful not to overmix.
Combine Gently: Add the banana purée to the whipped cream.
Using a spatula, gently fold the two mixtures together until just combined, being careful not to deflate the cream.
Add Flavorings: Gently stir in the lemon juice, ground nutmeg, and rum until evenly incorporated.
Chill and Serve: Spoon the mousse into 6 serving glasses or bowls.
Refrigerate for at least 1 hour to allow it to set before serving.