1. Prepare the raspberry puree: In a small saucepan, combine fresh raspberries, granulated sugar, and lemon juice. Cook over medium heat, stirring occasionally, until the berries break down and the mixture thickens slightly (about 5-7 minutes). Strain through a fine mesh sieve to remove seeds. Set aside and let cool.
2. Bloom the gelatin: Sprinkle the gelatin powder over cold water and let it sit for 5 minutes. Then, gently warm the gelatin mixture over low heat until fully dissolved (do not boil). Stir a few tablespoons of the cooled raspberry puree into the gelatin to temper it, then whisk this back into the rest of the puree.
3. Toast and chop pistachios: On a dry skillet over medium heat, toast the pistachios until fragrant and slightly golden (about 3-4 minutes). Let cool, then chop finely. Set aside about a tablespoon for garnish.
4. Whip egg whites: Using a clean, dry bowl, beat egg whites until stiff peaks form. Set aside.
5. Whip cream: In a separate bowl, whip the heavy cream with powdered sugar and vanilla extract until soft peaks form.
6. Make the mousse base: Fold the raspberry puree into the whipped cream gently until combined. Then, carefully fold in the egg whites, being mindful not to deflate the mixture. Finally, fold in the toasted pistachios.
7. Assemble the cakes: Place your pistachio sponge cake layer on a serving plate or cake ring. Pour or spoon the mousse evenly over the cake, smoothing the top with a spatula. Chill in the refrigerator for at least 4 hours, or preferably overnight, to allow the mousse to set properly.
8. Garnish and serve: Before serving, decorate with fresh raspberries and the reserved chopped pistachios for a fresh, vibrant finish.

From my experience, the key to a light mousse is gentle folding and patience. I remember Mama Lu saying, “Don’t rush the rhythm of the kitchen,” and that’s especially true here. Letting the mousse chill overnight makes all the difference in texture and flavor.