Heat the oven to 350°F. Lightly coat a 9-inch fluted tart pan with a removable bottom with nonstick cooking spray.

Combine the graham crackers, melted butter, and sugar in a food processor; pulse until fine crumbs form. Transfer the mixture to the prepared pan and press it evenly against the bottom and up the sides using a flat-bottomed glass or measuring cup. Set the pan on a baking sheet and bake for about 8 minutes, until lightly golden. Let the crust cool completely.

In a medium saucepan, combine the cranberries, sugar, cornstarch, and water. Bring to a boil over medium heat, stirring often. Lower the heat and simmer for 10 minutes, stirring frequently and crushing the cranberries with a spoon; a few small pieces can remain. Take the pan off the heat and cool the filling completely, or place it in the refrigerator to speed the process.

Put the chopped white chocolate in a medium bowl. In a small saucepan, warm the cream and butter over medium heat, stirring, until the butter melts and tiny bubbles appear around the edges. Pour the hot cream over the chocolate and whisk until silky and smooth.

Spread enough cranberry filling into the cooled crust to come about halfway up the sides, smoothing the top with a small offset spatula. You may have leftover filling; reserve it for another use.

Pour the white chocolate ganache over the cranberry layer, stopping when it reaches the top edge of the crust. Refrigerate the tart overnight so it sets completely. Garnish with sugared cranberries, plain cranberries, and rosemary sprigs just before serving.