In a medium saucepan, combine the strawberries, sugar, and lemon juice. Cook over medium-low heat, stirring occasionally, until the strawberries cook down to a pulp and the sauce becomes juicy, about 15 minutes. Pass the sauce through a fine-mesh sieve into a bowl, pressing down on the solids. Stir the vanilla bean into the purée. Set aside and let infuse for about 10 minutes.
Remove the vanilla bean and pour the sauce into a mason jar.