Preheat 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 and pulse until the mixture forms fine crumbs. Press the crumbs evenly into the bottom and up the sides of the prepared pan using a flat-bottomed glass or measuring cup. Put the tart pan on a baking sheet and bake until light golden brown, about 8 minutes. Allow to cool completely.

Put the cranberries, sugar, cornstarch, and water in a medium saucepan. Bring to a boil over medium heat, stirring often. Lower the heat and let it simmer for 10 minutes, stirring frequently and mashing the cranberries with a spoon until they soften; small pieces are fine. Take off the heat and cool completely, or pop the pan in the refrigerator to speed up the process.

Place the chopped white chocolate in a medium bowl. Warm the cream and butter in a small saucepan over medium heat, stirring occasionally, until the butter melts and small bubbles form around the edges. Pour the hot cream mixture over the chocolate and whisk until smooth and glossy.

Spoon enough cranberry filling into the cooled crust to come halfway up the sides; use a small offset spatula to smooth the top. You may have leftover cranberry filling, which can be saved for toast or ice cream. Pour the ganache over the cranberry layer, stopping when it reaches the top of the crust. Any extra ganache can be refrigerated for another use.

Refrigerate the tart overnight. Right before serving, garnish with sugared cranberries, fresh cranberries, and rosemary sprigs if desired.