Preheat oven to 350 degrees.  Line the bottom of a 9-inch tart tin (with a removable bottom) with a circular piece of parchment paper or grease a 9-inch pie pan.

In a medium mixing bowl, use your hands to combine cookie crumbs with the melted butter and the rum. Press into the bottom and up the sides of the prepared pan. Place the pan on a cookie sheet and bake for 10 minutes, or until set. Remove from oven, sprinkle crystallized ginger over the top of the crust, and set aside to cool.

Using an electric mixer with the whisk attachment, beat together the mascarpone cheese, sugar, lemon juice, and Limoncello until well blended and uniformly smooth. Fold in lemon zest with a spatula (it gets hung up on the whisk, so don’t use the machine).

In a separate mixing bowl, whip the cream until stiff peaks form. Fold whipped cream into the mascarpone mixture.

Drop dollops of the mascarpone-cream mixture onto the crystallized ginger and spread into an even layer. Arrange blueberries evenly over the filling, pressing down lightly so they stay in place. Cover with plastic wrap and refrigerate for at least one hour (up to eight hours).

To serve, gently and carefully remove sides of the tart pan. Lightly dust with powdered sugar and slice into 8 pieces.