Place 1/4 cup water in a cup and sprinkle gelatin on top to soften.

Mash raspberries in a bowl with a whisk or potato masher.

Strain the mashed raspberries into another bowl to remove seeds, yielding about 1 2/3 cups puree.

Add sugar and lemon juice to the raspberry puree and stir until sugar dissolves.

Microwave softened gelatin for 30 seconds on high power.

Measure 1/4 cup raspberry puree and set aside.

Whisk the microwaved gelatin into the remaining raspberry puree.

Whip the heavy cream until stiff peaks form.

Gently fold in one third of the raspberry-gelatin mixture into the whipped cream until nearly incorporated.

Fold in the remaining raspberry mixture in 2 more batches in the same manner.

Drizzle the reserved raspberry puree onto the mousse and fold once or twice for decorative streaks.

Scrape the mousse into a glass bowl, cover, and refrigerate until set, approximately 3 hours.

Serve the raspberry mousse in individual glass bowls or dishes.