Add frozen strawberries to a deep large saucepan. Cook over medium-low until some of the berries start to melt and release some juice, about 3 minutes. Increase the heat to medium-high and cook the strawberries, stirring frequently, until thick and gemlike, about 25 minutes. IMPORTANT: Measure the cooked filling, you should have only 2 cups. If you have more, return the filling to the saucepan and continue cooking until it is reduced to exactly 2 cups.
While the filling is cooking down, combine the gelatin, lemon juice and water in a small bowl. Set aside until gelatin is softened and the mixture is thick, about 5-minutes.
Add the softened gelatin mixture to the hot reduced strawberry mixture along with the sugar and pinch of salt. Return the berry mixture to a simmer. Cook for 2 minutes, the remove from the heat and transfer hot filling to a clean heat-proof bowl to cool, about 45 minutes.
(OPTIONAL) If topping with whole strawberries, place the 12 clean, dry strawberries in a small bowl. Spoon ¼ cup of the cooked and cooled strawberry mixture over the berries. Toss gently to coat. Remove the whole berries to a plate, flat side-down like they will be on top of the pie. Refrigerate until ready to serve. Return any extra filling back into the remaining mixture.
Fold the fresh sliced strawberries into the filling and spread evenly in the prepared pie crust. Refrigerate overnight or up to 24 hours. Add the whole strawberries in a circle in the middle just before serving.