Make the topping: In a small bowl combine strawberries, sugar, and mint leaves. Set aside. While sitting, the strawberries will release their delicious sweet juices.
Make the mousse: Place strawberries, water, and sugar in a saucepan and cook over medium heat until soft, about 3-5 minutes. Remove from heat and transfer to a large bowl. Using an immersion blender (or if you don’t have one, using your regular blender), puree until smooth. Stir in marshmallows and mix until dissolved. If not completely melted, use the blender again until mixture is smooth.
Using an electric mixer fitted with the whisk attachment, whisk heavy cream until it holds its shape (somewhere between soft to stiff peaks). Let strawberry mixture cool before adding whipped cream to prevent it from melting. When cool, fold half of the whipped cream into the strawberry mixture, then fold in the rest.
Evenly spoon or pipe mousse into serving dishes, then top with strawberry topping.
Cover and place in the fridge to firm for at least 2 hours before serving, or up to 3 days. The strawberry topping would look best at the same day it's made.