Make the cranberry sauce.
Place the cranberries, half the sugar, water, lemon juice and orange zest (if using) in a medium saucepan. Cook over medium heat until the berries pop and the mix thickens into a jammy sauce, about 8–10 minutes. Remove from the heat and cool to room temperature.

Bloom the gelatin.
Sprinkle the gelatin over the cold water in a small bowl and let it sit for 5 minutes to soften.

Incorporate gelatin into the fruit.
Warm the cooled cranberry mixture gently (on the stove or briefly in the microwave) until it’s just warm — not boiling. Stir the softened gelatin into the warm fruit until dissolved. Let the mixture cool again until it is no longer hot but still pourable.

Whip the cream.
In a chilled bowl, whip the heavy cream with the remaining sugar and vanilla until it holds stiff peaks. The cream should be firm enough to support the fruit swirl.

Fold everything together.
Gently fold the cooled cranberry-gelatin mixture into the whipped cream in several additions, using a spatula and delicate folding motions so you keep the air in the cream. Stop when the color and texture look even.

Fill the crust.
Spoon or pour the mousse into the prepared crust, smoothing the top lightly. Cover and transfer to the fridge.

Chill until set.
Refrigerate at least 4 hours, or overnight for the cleanest slices and fullest flavor.

Finish and serve.
Just before serving, pipe or spoon extra whipped cream, scatter a few sugared or fresh cranberries, dust with powdered sugar, and add rosemary sprigs for a festive touch.