Blueberries are one of summer's treats, and I have read repeatedly that they are the healthiest of fruits. So, enjoy the luscious taste and other benefits of this surprisingly easy to make, and spectacularly attractive, pareve dessert. You can use butter instead of pareve margarine in the crust, but I always use the margarine so that I can serve the tart, and eat the leftovers, any time. The crust is easy to make and, even with margarine, it is delicious. The tart can be served with vanilla ice cream, whipped cream, or lemon sorbet (a good pareve option), but I think it's best savored plain, to fully appreciate the fresh flavor of the blueberries.
Preheat oven to 375 degrees. Have ready a round tart pan, 9 or 9 1/2-inches in diameter, with a removable bottom.
Place the flour, sugar, and salt in a food processor. Cut the cold margarine into small pieces and scatter over the dry ingredients. Process until large moist crumbs form. Test by squeezing some of the dough with your fingers – it should hold together. Transfer the dough to the tart pan and press into the bottom and up the sides with your fingers so that the entire inner surface is evenly covered. Place the pan in the freezer for at least 15 minutes, until firm. Remove, prick the bottom and sides of the dough all over with a fork (to provide steam holes, preventing shrinkage). Bake in the oven until golden, approximately 15 minutes. Remove, set pan on a rack, and cool completely.
While the crust is baking and cooling, you can prepare to make the filling. (The cooled crust can sit on the counter for hours, if necessary, until you are ready to proceed with the filling.) Set aside one cup of the best-looking blueberries for the top of the tart. In a small bowl, mix together the cornstarch with 2 tablespoons water and set aside. Grate the lemon zest, juice the lemon, and set aside. Place the sugar and salt in a small bowl.
In a medium pot, bring 1/4 cup water and 1 1/2 cup blueberries to a boil over high heat. Reduce the heat and simmer, stirring occasionally, until the berries begin to break down. This should take about 3 to 5 minutes. Stir the cornstarch mixture until smooth, add to the pot, and stir immediately to combine. Quickly add the lemon zest, lemon juice, sugar, and salt. Stir and bring to a boil. Reduce heat and simmer, stirring continuously, just until the mixture begins to thicken, about 1 minute.
Remove the pot from the heat. Stir in the remaining 3 1/2 cups blueberries (not the reserved ones). Immediately pour the berry mixture into the cooled tart crust. Do this slowly so that you can control where the blueberries and juice end up. Mound the berries slightly in the center. Some of the juice may overflow the crust a bit, but that's fine.
Place the reserved blueberries on top of the filling, scattered all over. Press down on them lightly so that they adhere and don't roll off. Refrigerate the tart until cold, at least one hour. Serve within a day for best results, cold or at room temperature. (To serve, hold the pan with one hand, and with the other hand push the base of the pan up through the top of the ring. You can place the tart with the base on a serving platter, or gently remove the tart from the base using a wide spatula and sliding the tart onto the platter.)
Jamie Stolper is the Food Editor of ShalomBoston.com.