Bloom the gelatin by sprinkling it over cold water and letting it sit for 5 minutes.

Cook raspberries with sugar and lemon juice over low heat for about 10 minutes, mashing until smooth.

Strain the raspberry mixture through a fine-mesh sieve into a bowl and let cool for 5-10 minutes.

Mix bloomed gelatin into the warm raspberry syrup until dissolved.

Let the raspberry mixture cool to room temperature for about 15-20 minutes.

Whip chilled heavy cream with powdered sugar until stiff peaks form.

Fold in 1/4 of whipped cream into the raspberry mixture, then gently fold in the rest until just combined.

Spoon mousse into serving dishes, smooth tops, cover, and refrigerate for at least 1-2 hours until set.