Filling 1 8-ounce container mascarpone cheese 6 ounces cream cheese, room temperature 1/4 cup sour cream 1/4 cup sugar 1 tablespoon grated lemon peel 1/4 teaspoon vanilla extract Topping 4 to 5 small nectarines, halved, pitted, cut into thin slices 1/4 cup peach jam, warmedPreparation For crust: Preheat oven to 350°F. Finely grind gingersnaps in processor. Add butter and blend until crumbs are evenly moistened. Press mixture over bottom and up sides of 9-inch-diameter tart pan with removable bottom. Bake crust until color darkens, pressing sides with back of spoon if beginning to slide, about 8 minutes. Cool completely.
For filling: Beat first ingredients in medium bowl until smooth. Spread filling in prepared crust. Cover loosely and refrigerate at least 2 hours and up to 1 day.
For topping: Overlap nectarine slices atop filling in concentric circles. Brush with jam. Serve, or refrigerate up to 6 hours.