In a medium saucepan, combine fresh cranberries, ½ cup sugar, ½ cup water, lemon juice, and optional orange zest. Cook over medium heat for about 10 minutes, stirring occasionally, until the cranberries burst and the mixture thickens slightly. Remove from heat and let cool to room temperature.

While the cranberry sauce cools, sprinkle 2 teaspoons of gelatin over 3 tablespoons of cold water in a small bowl. Let it sit for about 5 minutes until it blooms and becomes spongy.

Gently heat the cooled cranberry mixture again, stir in the softened gelatin until fully dissolved. Remove from heat and let it cool again, keeping it liquid.

In a large mixing bowl, pour in 1 cup of chilled heavy whipping cream. Beat the cream on medium speed until it begins to thicken. Gradually add the remaining ½ cup sugar and 1 teaspoon of vanilla extract, then whip until stiff peaks form.

Gently fold the cooled cranberry mixture into the whipped cream using a spatula. Be careful not to deflate the mixture.

Once combined, pour the mousse into the prepared graham cracker crust, smoothing the top. Chill and set in the refrigerator for at least 4 hours or overnight.

Before serving, top your pie with extra whipped cream, dusted cranberries, or fresh rosemary. Slice into portions and serve chilled.