How to Make It
Preheat the oven to 325°. Line the bottom of a 9-by-13-inch baking pan with parchment paper and lightly spray the bottom and side of the pan with baking spray.
In a bowl, sift the flour with the ground ginger, baking soda, salt, cloves and nutmeg. In a large bowl, beat the molasses with the sugar and butter at medium speed until fluffy. Beat in the egg and the fresh and crystallized ginger. At low speed, beat in the dry ingredients. Gradually beat in the water. Spread the batter in the prepared pan.
Bake the gingerbread for about 20 minutes, rotating the pan once, until a tester inserted in the center comes out clean. Transfer the gingerbread to a rack to cool for 30 minutes, then turn it out and let it cool completely, 1 hour. Refrigerate the gingerbread until firm, about 1 hour. Peel off the parchment paper and cut the gingerbread into 1-inch cubes.
In a small bowl, sprinkle the gelatin over 2 tablespoons of water and let stand for 5 minutes. In a medium bowl, beat the egg yolks with the sugar until fluffy.
In a medium saucepan, combine 1/2 cup of the cream with the milk, zest and salt and bring to a simmer. While whisking, drizzle the hot milk mixture into the egg yolks until incorporated. Transfer the mixture to the saucepan and cook over low heat, stirring, until the custard is thickened and coats the back of a spoon, about 10 minutes. Remove from the heat. Add the gelatin and white chocolate and stir until melted. Strain the custard into a medium metal bowl set in a bowl of ice water and cool to room temperature, stirring, about 2 minutes. Remove the bowl from the ice bath.
In a large bowl, beat the remaining 2 1/2 cups of heavy cream until soft peaks form. Fold the whipped cream into the custard. Refrigerate the mousse until chilled, about 2 hours.
In a medium saucepan, combine the sugar with 3 tablespoons of water and cook over moderate heat, stirring, until the sugar dissolves; brush down the side of the pan with a wet pastry brush. Cook undisturbed until a medium-amber caramel forms, about 5 minutes. Carefully add the pomegranate juice and lemon juice and simmer until slightly thickened, about 5 minutes. Let cool to room temperature.
In an 8-inch, 2-quart trifle bowl, make 4 layers of the gingerbread, mousse and caramel. Garnish with the pomegranate seeds and crystallized ginger slices and serve.