Pre-heat oven to 375 - place rack on the lower 1/3 of the oven.
1 1/4 cups of flour
1 stick of softened unsalted butter
2 Tablespoons of sour cream to start
1/2 teaspoon sugar
1/4 teaspoon salt.
Place all ingredients in food processor and pulse. If the dough doesn't form a ball fairly quickly, add more sour cream - up to one additional Tablespoon. Either pat the dough into a 10 inch tart pan with lightly floured hands or roll the dough between two lightly floured sheets of parchment or plastic wrap and place the dough carefully into the tart pan. Prick the dough with fork tines several times to keep dough flat while baking. Cover with plastic wrap and chill for at least 30 minutes. Lightly spray parchment paper and lay sprayed side over dough in the pan and fill pan with weights or beans. Bake for 20 minutes. Remove the parchment liner and weights, reduce the heat to 350 and return the tart pan to the oven and bake until light golden brown, about 10 to 15 minutes more. (Note - parchment paper works best if crumpled before using)
FILLING: (fills 10" x 1" tart pan)
1/3 cup of sugar
1 Tablespoon of cornstarch
1/2 teaspoon of finely grated lemon zest
1/4 teaspoon salt
12-15 Italian plums cut into 1/8's
2 Tablespoons of plum jam
Cut the plums in half remove pits. Cut into quarters then cut quarters in half. In a bowl, whisk the sugar, cornstarch, salt and lemon zest. Add the sliced Italian plums and toss to coat evenly. Cover and refrigerate over night. Drain the plums and put the juice in a small sauce pan and reduce the juice to a thick syrup. Set aside. Heat the plum jam to spreadable constancy and brush the bottom and sides of the tart shell with a thin coat of jam. Spread the reduced syrup over the plum jam, coating the bottom of the tart shell. Arrange the plums around the tart shell starting on the outside, placing them cut side down (it is OK if they overlap slightly). Tuck in as many as you can in a petal design. If there is a Tablespoon of reduced syrup remaining, pour that over the plums and sprinkle with a tablespoon of sugar.
Lower the oven to 350°F. Bake until juices are bubbling and crust is golden brown, approximately 30 minutes. Cool on a rack before removing tart ring.