For crust six 5- by 2 1/2-inch graham crackers 1 1/4 cups coarsely crushed amaretti (Italian almond macaroons, about 3 ounces) 3 tablespoons sugar 3 tablespoons unsalted butter, melted For filling 1 1/2 cups mascarpone cheese (about 3/4 pound) at room temperature 1/2 cup sugar 2 large eggs 1 tablespoon fresh lemon juice 2 teaspoons freshly grated lemon zest 1 teaspoon vanilla extract 1/4 teaspoon almond extract 3 tablespoons all-purpose flour 1/4 teaspoon salt For topping 2 pounds nectarines (about 8) 2 tablespoons sugar, or to taste If you cannot find amaretti you can use graham crackers.