Stir the mascarpone and water in a bowl until smooth, season. Add the potatoes and all the other ingredients up to and including the pepper, mix well. Spoon the mixture into the dishes. Sprinkle the cheese on top of the gratins.
Bake for approx. 25 mins. in the lower half of an oven preheated to 220°C. Remove the gratins from the oven, drizzle with balsamic and garnish with the reserved onion greens.