Preheat the oven to 425 degrees F
Roll out one of the discs of dough into a circle that will fit into the base and the sides of an 8-inch tart pan (you can also use an 8-inch pie plate). Flour the surface and rolling pin before rolling.
Fold the dough into half and transfer to the tart pan. Unfold it and fit it into the pan, pressing into the sides and the bottom to make sure it cover the pan uniformly. Use a rolling pin to roll over the top of the fluted pan and remove any overhanging bits of dough.
Place the tart pan in the refrigerator while you make the bottom crust.
To make the bottom crust, roll out the second disc of dough. Measure the diameter of the top of the tart pan and, leaving a 1-inch border, cut out little shapes into the dough using a cookie cutter. I used a heart, but stars of little circles cut with the cap of a bottle, would look beautiful too.
Mix the cornstarch thoroughly into the cranberry filling.
Remove the tart pan from the refrigerator and pour the cranberry filling into it.
Carefully, place the top crust over the filling. Use a knife or your fingers to press over the edges and remove any overhanging bits of dough so you have a neat edge.
With a drop of water dabbed to the bottom, paste the little hearts or whatever shapes you cut out of the dough along the rim of the tart pan, pressing in slightly to ensure they stick.
I like to brush on a mixture of 1 tsp maple syrup, 1 tsp vegetable oil and 1 tsp nondairy milk on the top crust to get an extra-golden and shiny glaze on the tart. You can skip this step, but your tart won't look as golden.
Bake the tart in the preheated oven for 50 minutes, until the top crust is golden and the cranberry juices are bubbling.
Remove the tart and let it cool 10 minutes. If using a pie plate, you can let the pie cool in the plate before serving. If you baked the tart in a pan with a removable base, separate the tart from the base at this point and let the tart cool thoroughly on a rack before serving.