Preheat your oven to 350°F (175°C) and grease a 9-inch springform pan, lining the bottom with parchment paper. In a food processor, combine the all-purpose flour, powdered sugar, unsalted butter, and salt. Pulse until the mixture resembles coarse crumbs. Firmly press this mixture into the bottom of the prepared pan and bake for 15-18 minutes, or until the edges are lightly golden. Allow the crust to cool completely before adding the mousse.

In a small bowl, bloom the gelatin in water for 5 minutes. Meanwhile, gently warm fresh lemon juice in a saucepan over low heat, then add the bloomed gelatin, stirring until completely dissolved. Once dissolved, set the mixture aside to cool to room temperature. In a mixing bowl, whip the heavy cream, sugar, and lemon zest until stiff peaks form. In a separate bowl, beat the cream cheese until smooth, then fold the cooled lemon mixture into the cream cheese, followed by the whipped cream.

In a medium saucepan, combine fresh blueberries, granulated sugar, and fresh lemon juice. Cook over medium heat for 5-7 minutes, stirring occasionally, until the blueberries soften and begin to release their juices. Remove from heat and let the compote cool to room temperature.

Once the shortbread crust is cool, spread a layer of lemon mousse evenly over the crust, smoothing it out with a spatula. Follow with a layer of the cooled blueberry compote, spreading it gently to cover the mousse. Repeat the layers, finishing with a layer of blueberry compote on top.

Cover the assembled Lemon Blueberry Shortbread Mousse Cake with plastic wrap and refrigerate it for at least 4 hours, or until set.

Once set, carefully run a knife around the edge of the springform pan to loosen the sides. Gently remove the springform and slice the cake into wedges. Serve chilled.