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

Pulse the graham crackers, melted butter, and sugar in a food processor until the mixture forms fine crumbs. Turn the crumbs into the prepared pan and press them evenly into the bottom and up the sides with a flat-bottomed glass or measuring cup. Set the pan on a baking sheet and bake for 8 minutes, until pale golden. Set aside to cool completely.

Combine the cranberries, sugar, cornstarch, and water in a medium saucepan over medium heat. Bring to a boil, stirring often, then lower the heat and simmer for 10 minutes. Stir frequently and use a spoon to crush the berries; some chunks can remain. Remove from the heat and cool completely, or transfer to the fridge to chill faster.

Place the chopped white chocolate in a medium bowl. Warm the cream and butter in a small saucepan, stirring often, until the butter melts and small bubbles appear at the edges. Pour the hot cream over the chocolate and whisk until the ganache is smooth and glossy.

Spoon enough cranberry filling into the cooled crust to fill it halfway up the sides, then smooth it with a small offset spatula. Pour the ganache over the filling, stopping when it reaches the top edge of the crust. Reserve any leftovers in the fridge for another use.

Refrigerate the tart overnight. Just before serving, top with sugared cranberries, plain cranberries, and rosemary sprigs.