How to Make It
In a blender, puree the sorrel with 3/4 cup of the chicken stock until smooth. In a small bowl, sprinkle the gelatin over the remaining 1/4 cup of chicken stock and let stand until softened, about 2 minutes. Microwave the gelatin at medium power just until melted, 5 to 10 seconds. Add the gelatin mixture to the blender along with 2 tablespoons of the lemon juice and the red wine vinegar. Season with salt and white pepper and puree until very smooth. Add 1 cup of the heavy cream and pulse just until very lightly whipped, about 1 minute. Transfer the sorrel mousse to a measuring cup and pour into 6 small glasses. Refrigerate the mousse until set, about 4 hours.
Meanwhile, in a bowl, whisk the remaining 1/4 cup of cream and 1 tablespoon of lemon juice with the crème fraîche to soft peaks. Season with salt and white pepper. Dollop the lemon cream on the mousse and top with the caviar. Serve chilled.