Combine strawberries, 1 tablespoon granulated sugar, and lemon juice in a food processor. Blend until completely smooth, about 1 minute. Press through a fine-mesh sieve into a bowl to remove seeds; you should have about 1 cup puree. Discard seeds.

Sprinkle gelatin over cold water in a small bowl. Let stand 5 minutes to soften. Microwave for 10-15 seconds until liquid and warm to the touch, not hot. Whisk immediately into the strawberry puree until fully incorporated. Set aside at room temperature.

In a large bowl, beat heavy cream, powdered sugar, vanilla, and remaining 2 tablespoons granulated sugar with an electric mixer on medium-high speed until soft peaks form, about 2-3 minutes. The cream should hold its shape but still curl at the tip when you lift the beaters.

Check that the strawberry-gelatin mixture is no longer warm (lukewarm is fine). Pour it over the whipped cream. Gently fold with a rubber spatula, cutting through the center and turning the bowl, until no streaks remain and the mixture is a uniform pale pink. Work quickly but gently to maintain volume.

Divide the mousse among 6 ramekins or glasses, filling about three-quarters full. Smooth the tops with a small offset spatula or the back of a spoon. Refrigerate uncovered for at least 2.5 hours, preferably 4, until completely set and firm to the touch.

Top each mousse with fresh strawberry halves and a mint leaf if desired. Serve chilled. The mousse will keep, covered, for up to 2 days in the refrigerator.