Make the dough. Place the flour, sugar, and salt in a food processor, process to combine. Cut the butter into small chunks, add to the mixer, and pulse until the mixture is coarse and crumbly. Pulse in the yogurt, then add the water 1 tablespoon at a time until the flour holds together when pinched, but is still slightly crumbly. Turn the mixture out onto a piece of plastic wrap, and use the edges of the plastic wrap to form it into a ball. Refrigerate for about an hour.
Make the filling. Place the filling ingredients in a mixing bowl, and thoroughly combine. Let sit until the sugar is dissolved and the mixture is wet, stirring occasionally.
Preheat the oven to 375°F. Roll the pastry dough on a floured surface into 1/4 inch thick rectangle. (Or roll the pastry into a rectangle as thin as you can, then cut into 6 sections and roll each section 1/4 inch thick.) Carefully press dough into the tart cavities and crimp the edges. Fill each tart with about 1/3 of the rhubarb mixture. Bake for 30 – 35 minutes until the filling is bubbling and the edges of the crust browned. Serve at room temperature.