In a small saucepan, combine raspberries, granulated sugar, and lemon juice. Cook over medium heat for about 5 minutes, stirring occasionally until the raspberries break down. Remove from heat and let cool slightly.

In a separate bowl, sprinkle gelatin over water and let it sit for 5 minutes to bloom. Then, heat in the microwave for about 10 seconds until dissolved.

In a large mixing bowl, whip the heavy cream and vanilla extract until soft peaks form.

Gently fold in the mascarpone cheese, powdered sugar, and salt until well combined.

Add the dissolved gelatin and half of the raspberry mixture to the cream mixture, folding gently until combined.

Divide the mousse evenly into serving cups or small cake molds. Refrigerate for at least 2 hours to set.

Once set, top each mousse with the remaining raspberry mixture and sprinkle with chopped pistachios before serving.