Pre-heat the oven to 350 degrees. Spray a 9" fluted tart pan with removable bottom with non-stick cooking spray.

Place the graham crackers, melted butter and sugar into a food processor and pulse till fine crumbs form. Pour into your prepared pan and use a flat-bottomed glass or measuring cup to press the crumbs firmly into the bottom and sides of the pan. Place on a baking sheet and bake till light golden brown, about 8 minutes. Remove from the oven and let cool.

Place the cranberries, sugar, cornstarch and water into a medium saucepan. Bring to a boil over medium heat, stirring frequently. Reduce the heat and simmer for 10 minutes, stirring frequently and breaking down the cranberries with your spoon (there will be some pieces remaining, which is fine). Remove from heat and cool completely (you can pop the pan in the fridge to speed things up if you like).

Place the chopped chocolate into a medium bowl. Heat the cream and butter in a small saucepan, stirring frequently, till the butter is melted and small bubbles appear around the edges. Immediately pour the cream over the chocolate and whisk till smooth.

Spread enough of the cranberry filling into the tart to come about halfway up the sides (you may have some leftover filling, which is delicious on toast or over ice cream!). Use a small angled spatula to smooth and level. Pour the ganache over, stopping when it comes level with the top of the crust (any leftover ganache can be stored in the fridge - check the notes section on ideas to use!).

Refrigerate overnight. Before serving, garnish with sugared cranberries and rosemary sprigs if using.