How to Make It
In a food processor, combine the flour, sugar and salt and pulse to blend. Add the butter and pulse until the mixture resembles coarse meal. In a small bowl, beat the egg with the water, then add it to the processor and pulse until combined. Transfer the dough to a work surface and gather it into a ball. Divide the dough into 6 equal pieces and flatten each piece into a 4-inch disk. Wrap each disk in plastic and refrigerate for 30 minutes or up to 1 day.
Preheat the oven to 350° and lightly butter six 4-by-1-inch ring molds or tartlet pans with removable bottoms. If using ring molds, line a baking sheet with parchment paper. On a lightly floured work surface, roll the pastry disks into 1/8 -inch-thick rounds and gently ease them into the molds. Trim off any overhang. Bake the pastry shells for about 30 minutes, or until golden all around and cooked through. Let cool completely before unmolding.
Melt half of the butter in a medium skillet. Add the 4 mashed bananas and the honey and cook over moderately high heat, stirring, until hot to the touch and very smooth, about 2 minutes. Add 2 tablespoons of the lemon juice and the nutmeg, than transfer the puree to a bowl to cool completely. In a medium bowl, whip the heavy cream with the confectioners' sugar until firm. Fold the banana puree into the whipped cream and refrigerate.
Up to 1 hour before serving, slice the remaining 4 bananas 1/3 inch thick and toss with the remaining 2 tablespoons of lemon juice. Melt the remaining 2 tablespoons of butter in a medium skillet. Add the granulated sugar and cook over moderate heat, stirring, until an amber caramel forms, about 5 minutes. Remove the skillet from the heat and at arm's length, add the rum. Stir until smooth. Add the sliced bananas and stir to coat with the caramel.
Spoon the banana cream into the pastry shells and arrange the caramelized banana slices decoratively on top. Transfer the tartlets to dessert plates. Spoon any leftover caramel sauce over the bananas and onto the plates and serve.