Mash strawberries in a bowl or use a blender/food processor to make about one cup of juice and pulp. Place in a small saucepan with jam and stir together.
In a small cup, mix the cornstarch and sugar together, then add to the saucepan and mix.
Begin heating on medium-low, stirring frequently. Bring mixture to a boil, then cook, stirring constantly, for at least one minute to thicken. Remove from heat and set saucepan aside to cool.
Transfer sauce to a small bowl or jar and refrigerate, covered, until ready to use. Filling will thicken as it cools.
