1. Make mousse: Combine lemon juice and 1 Tbsp. water in a small saucepan. Sprinkle gelatin over and let stand for 10 minutes.
2. In a food processor, blend strawberries and 1/2 cup sugar until smooth. Strain through a fine-mesh sieve into a large bowl; discard solids. Place pan with gelatin over very low heat and warm, stirring to dissolve gelatin. Whisk into strawberry mixture.
3. Using an electric mixer at medium-high speed, beat cream until it forms stiff peaks. With a large flexible spatula, fold 1/3 of whipped cream into strawberry mixture until well combined, then fold in remaining cream until no white streaks remain. Cover with plastic (pressing it onto surface of mousse to prevent a skin from forming) and refrigerate until well chilled, at least 1 hour.
4. Make meringues: Preheat oven to 250ºF, place a rack on top rung and line a baking sheet with parchment paper. In a heatproof bowl, combine egg whites with remaining 1/2 cup sugar. Set bowl over a saucepan of simmering water (be sure bottom of bowl does not touch water). Heat, whisking constantly, until mixture is very warm to touch, about 5 minutes. Transfer to an electric mixer and beat at high speed until mixture has quadrupled in volume, holds stiff peaks and is glossy, about 5 minutes.
5. Spoon meringue into a piping bag and pipe 16 1 1/2-inch hearts onto baking sheet. Bake for 30 minutes. Turn off oven, prop open oven door with handle of a wooden spoon and let meringues sit, undisturbed, until completely dry and no longer chewy, at least 4 hours or overnight. Serve mousse in bowls, each garnished with 2 or 3 hearts; pass remaining meringues.