In a medium saucepan, combine fresh cranberries, 1/2 cup sugar, 1/2 cup water, and lemon juice. Cook over medium heat for about 10 minutes, until cranberries burst and mixture thickens.

In a small bowl, sprinkle gelatin over 3 tablespoons cold water and let it soften for about 5 minutes.

Gently warm the cranberry mixture until just warm, then stir in the softened gelatin until dissolved. Let cool to room temperature.

In a large mixing bowl, whip 1 1/2 cups chilled heavy cream with the remaining 1/2 cup sugar and a teaspoon of vanilla extract until stiff peaks form, about 3-5 minutes.

Gently fold the cooled cranberry-gelatin mixture into the whipped cream using a spatula.

Pour the mixture into the prepared graham cracker crust and smooth the top.

Cover and refrigerate for at least 4 hours or overnight until set.

Before serving, top with additional whipped cream, sugared cranberries, and optional rosemary sprigs.