nectarine, mascarpone and gingersnap tart

INGREDIENTS

• 37 gingersnap cookies, coarsely broken (about 9 ounces; about 3 1/4 cups plus 2 tablespoons of pieces)
• 6 tablespoons unsalted butter, melted
• 1 8-ounce container mascarpone cheese
• 6 ounces cream cheese, room temperature
• 1/4 cup sour cream
• 1/4 cup sugar
• 1 teaspoon grated lemon peel
• 1/4 teaspoon vanilla extract
• 1 tablespoon finely chopped crystallized ginger (optional)
• 4 to 5 small nectarines, halved, pitted, cut into thin slices
• 1/4 cup peach jam, warmed
• 2 tablespoons finely chopped crystallized ginger (optional)