Combine lemon juice and water in a small saucepan.

Sprinkle gelatin over the mixture and let stand for 10 minutes to bloom.

In a food processor, blend strawberries and 1/2 cup sugar until smooth.

Strain the strawberry mixture through a fine-mesh sieve into a large bowl; discard the solids.

Place the saucepan with gelatin over very low heat and warm, stirring to dissolve the gelatin.

Whisk the dissolved gelatin into the strawberry mixture.

Using an electric mixer at medium-high speed, beat heavy cream until stiff peaks form.

With a large flexible spatula, gently fold 1/3 of the whipped cream into the strawberry mixture until well combined.

Fold in the remaining cream until no white streaks remain.

Cover the mousse with plastic wrap, pressing it onto the surface to prevent a skin from forming.

Refrigerate the mousse until well chilled, at least 1 hour.

Preheat oven to 250°F (120°C). Place a rack on the top rung and line a baking sheet with parchment paper.

In a heatproof bowl, combine egg whites with the remaining 1/2 cup sugar.

Set the bowl over a saucepan of simmering water (ensure the bottom of the bowl does not touch the water).

Heat, whisking constantly, until the mixture is very warm to the touch, about 5 minutes.

Transfer the mixture to an electric mixer and beat at high speed until it has quadrupled in volume, holds stiff peaks, and is glossy, about 5 minutes.

Spoon the meringue into a piping bag fitted with a small round tip.

Pipe 16 heart shapes onto the prepared baking sheet.

Bake the meringues for 30 minutes.

Turn off the oven, prop open the oven door with the handle of a wooden spoon, and let the meringues sit undisturbed until completely dry and no longer chewy, at least 4 hours or overnight.

Serve the mousse in bowls, each garnished with 2 or 3 meringue hearts; pass remaining meringues.