Combine mashed strawberries and water in a saucepan; bring to a boil and simmer for about 3 minutes.

Strain juice from cooked strawberries and add enough cold water to make 1 cup of juice. Discard pulp.

Combine sugar, cornstarch, lemon juice, and salt in a saucepan; stir strawberry juice into the sugar mixture. Bring to a boil, stirring constantly, until clear and thick, 3 to 5 minutes. Remove from heat and cool slightly.

Stir sliced strawberries into strawberry sauce until evenly coated. Chill before serving to allow filling to set completely.