Puree the raspberries in a food processor until completely smooth. Strain through a fine-mesh sieve into a medium bowl, discarding the solids. Add the gelatin to the raspberry puree and stir; set aside to bloom for at least 7 minutes.
Combine the raspberry-gelatin mixture, heavy cream and sugar in a medium saucepan over medium-low heat. Cook, stirring occasionally with a rubber spatula, until the sugar has dissolved completely, about 5 minutes. (Do not use a whisk.) Strain the custard through a fine-mesh sieve into a large bowl, pressing it through with a rubber spatula. Stir in the lemon juice.
Place two 6-cavity silicone rose molds (about 2 3/4 by 1 1/2 inches per cavity) on a baking sheet. Pour the custard into the cavities, filling them to the top. Cover and freeze overnight.
Sprinkle a few rose petals on each plate. Unmold the frozen panna cottas onto the plates, then let thaw completely before serving.