Dough: In the food processor, combine flour, sugar, salt and lemon zest. Pulse to blend. Add the butter and pulse until crumbly. Pour the cold water through the hole in the top of the machine and pulse only to combine. Form into an 8-inch long cylinder (about 2 inches in diameter) and refrigerate for at least 1 hour.
Filling: Heat a large sauté pan over high heat. Place a baking sheet next to the stove. When the sauté pan is fairly hot, add the butter for the filling. When it melts and foams slightly, add the strawberries. Toss to coat with the butter and add 1/4 cup sugar, lemon zest and juice. Toss to blend and allow some of the liquid to escape from the berries and reduce. After 2 minutes, sprinkle with cornstarch, reduce heat to medium and cook, stirring for an additional 2 minutes. Transfer to the baking sheet. Spread the strawberry mixture out so it cools faster and refrigerate until very cool.
Preheat the oven to 375°F.
Remove the dough from the refrigerator and cut into 8 equal pieces. Press each disk with your hand to flatten and using a rolling pin, roll each piece into a circle that is about 6-inches in diameter and about 1/8 inch thick. Make sure you flour lightly under and on top of the dough as you roll it. Trim edges with a sharp paring knife or circular pizza cutter if you want a cleaner edge on the crostata.
Meanwhile, remove strawberry mixture from refrigerator and transfer to a fine strainer placed over a bowl. Strain strawberries so that almost no liquid remains with the berries, being sure not to crush the fruit.
Arrange the tart rounds in a single layer on a baking sheet. Place 1 tablespoon walnuts onto the center of each round. Divide the cooked strawberries evenly among the dough circles. Fold the sides up tightly around the fruit to form a purse, making the fruit the center, they will open slightly during baking. Cover the opening left after folding with a few more walnuts. Refrigerate circles if they become too soft to handle.
Place the baking sheet in the center of the oven and bake until golden brown, 20 to 25 minutes. Top with any remaining walnuts. Sprinkle with sugar for added texture. Let cool on baking sheet for 5 minutes then transfer to serving plates using a wide spatula. Serve immediately.
Note: Use strained thickened strawberry liquid as a sauce with the tarts or with ice cream. Stir in 1 teaspoon Grand Marnier, if desired.