1. In a small bowl, whisk together the sugar, cornstarch, and pectin until evenly dispersed.
2. In a 2-quart saucepan, combine the water and lemon juice. Slowly whisk the dry ingredients into the water until combined. Place over medium heat and cook, stirring constantly, until the mixture thickens and reaches 170°F.
3. Add the frozen cherries to the saucepan and continue cooking, stirring gently, until the mixture reaches 180°F. Remove from the heat.
4. Transfer the filling to a heatproof bowl and cover with plastic wrap. Set aside to cool for at least 1 hour before using.