Make the Shortbread Crust:
- Add the flour, almond flour, powdered sugar, and salt to the bowl of a food processor and pulse until combined. Add in the butter and pulse 5-7 times, or until the butter is broken down into tiny pebbles.
- With the motor running, add in the egg and let the dough process until it forms a cohesive ball around the blade, about 1 minute.
- Remove the dough from the processor and shape it into a disk. Wrap it in plastic wrap, then chill it for 30 minutes.
Prepare the Plums:
- Preheat the oven to 400ºF and place an oven rack in the middle part of the oven.
- In a roasting pan, stir together the plums, cane sugar, vanilla bean and seeds, orange juice, and salt.
- Roast in the preheated oven for about 40 minutes, stirring about every 10-15 minutes, until the plums have softened and the juices have thickened to a light syrup consistency.
- Set a fine-mesh strainer over a bowl.
- Carefully pour the plums into the strainer, letting most of the juices strain out. Do not push down on the plums, as this will cloud up the sauce. Transfer the sauce to a small bowl and set aside in the refrigerator until ready to serve.
- Remove the vanilla beans and purée the plums and remaining juice using an immersion blender (or regular blender). Set the fine-mesh strainer back over the bowl and strain the purée, leaving behind and discarding any bits of skin.
Make the Mascarpone Filling and Assemble the Pie:
- Place the chilled dough on a flour-dusted work surface.
- Roll out the pie dough to an 11- to 12-inch circle and line it into a 9-inch tart pan.
- Line with parchment and add pie weights (or dry rice or beans or lentils), pushing the pie weights to the edges.
- Bake for 20 minutes, then remove from the oven and carefully remove the parchment and pie weights. Return to the oven and bake for an additional 5 minutes, or until the pastry is fully cooked through. Set aside until ready to use.
- Lower the oven temperature to 325ºF.
- In a bowl, mix together the mascarpone, crème fraîche, and cane sugar until smooth. Then, whisk in the egg, vanilla extract, and nutmeg.
- Add ¼ cup of the mascarpone filling to the plum purée and stir to combine.
- Spoon a few dollops of the plum purée filling into the pie crust. Then, pour the mascarpone filling on top. Spoon on dollops of the remaining plum mixture. Using a knife or chopstick, gently swirl through all layers of the filling to achieve a marbled effect.
- Bake the pie for 40-45 minutes, or until the pie reaches a temperature of 180ºF and the outer edges have started to puff up but the center is still slightly wobbly.
- Remove the pie from the oven and transfer to a wire rack to cool. Let cool fully at room temperature before placing in the refrigerator for at least 6 hours to finish firming up.
- Immediately before serving, drizzle over the reserved plum sauce.