1. Preheat the oven to 350°F.
2. Finely grind the sugar cookies in a food processor. Add the melted butter, and blend until the crumbs are evenly moistened. Press the mixture over the bottom and up the sides of a 9-inch tart pan with a removable bottom. Bake until the color darkens, pressing the sides with the back of a spoon if they begin to slide, about 8 minutes. Remove from the oven, set aside on a wire rack, and allow to cool completely.
3. Combine the mascarpone, cream cheese, sour cream, sugar, vanilla extract, and almond extract in a medium bowl, and beat with a handheld electric mixer on low speed until smooth. Spread this filling in the cooled crust. Cover loosely, and refrigerate until the filling is set, for at least 2 hours and up to 1 day.
4. Carefully arrange the nectarine slices on top of the chilled filling, fanning the slices out in concentric circles to cover as much of the tart as possible. Brush with the warm jam.
5. Serve immediately, or cover and refrigerate for up to 6 hours before serving.
Yield: 8 servings