Make the crust

1. In a mixing bowl, combine the millet flour, almond flour, tapioca flour, sweetener, and salt. Add the cold, cubed butter and rub it in with your fingertips until the mixture resembles coarse sand. As the dough tends to become a little sticky, you may want to use just one hand and keep the other clean.
2. Add the beaten egg and mix gently until the dough comes together, being careful not to overmix. If the dough feels too soft, you may add a little more millet flour, but keep in mind that it will firm up in the refrigerator.
3. Shape the dough into a disc, wrap it in plastic wrap, and refrigerate for 30 minutes.
4. Roll out the chilled dough between two sheets of parchment paper lightly dusted with millet flour, forming a circle approximately 9 inches (23 centimeters) in diameter—large enough to cover the bottom and sides of the tart pan. Remove the top sheet of parchment paper, then use the remaining sheet to lift and invert the dough over an 8-inch (20-centimeter) tart pan with a removable bottom. Gently peel away the parchment paper and fit the dough into the bottom and sides of the pan. If the dough breaks around the edges, simply patch it with your fingers, making sure the sides are covered evenly. Prick the bottom of the crust with a fork and freeze for at least 30 minutes.

Prebake the crust

1. While the dough chills, preheat the oven to 170°C (340°F) with a baking sheet on the middle rack. Halve and pit the plums, sprinkle them with one teaspoon of sweetener, toss, and set them aside.
2. Once the crust is frozen, place the tart pan on the baking sheet and blind bake it for 12 minutes, or until it looks set and just begins to turn golden. Remove it from the oven and let it cool.

Make the filling

1. Increase the oven temperature to 180°C (355°F). Leave the sheet pan in the oven.
2. In a mixing bowl, whisk together the whipping cream, eggs, fine sweetener, tapioca starch, vanilla, almond extract, and optional rum until smooth. You can use an electric mixer on low speed, but avoid overmixing. The custard will be quite thin and runny; this is how it should be.

Assemble and bake

1. If the plums are not very sweet, sprinkle them with 1 teaspoon of granulated monk fruit and allulose sweetener and toss them.
2. Arrange the plum halves evenly, cut-side down, over the cooled crust.
3. Pour the custard filling evenly over the plums. Using a small knife and fork, gently lift the plum halves to help the custard flow underneath and fill any gaps. Bake for about 35 minutes, or until the top is golden brown and the custard is bubbling lightly but set around the edges.
4. Let the tart cool at room temperature, then refrigerate it for at least 1 hour before serving. Carefully remove it from the pan, transfer it to a serving platter, dust with allulose, slice, and enjoy.