In a mixing bowl, combine the sliced strawberries and 1/4 cup of the granulated sugar. Toss gently to coat the strawberries evenly. This will help draw out the juices from the strawberries, enhancing the flavor of the filling.

In a medium saucepan, combine the remaining 1/4 cup of granulated sugar, cornstarch, salt, and water. Stir until well mixed. This mixture will serve as the base for your filling.

Place the saucepan over medium heat and bring the mixture to a gentle boil, stirring constantly. This step is crucial as it activates the cornstarch, thickening the filling.

Once the mixture reaches a boil and thickens (about 2-3 minutes), reduce the heat to low and add the prepared strawberries and lemon juice. Stir gently to combine, allowing the strawberries to soften and release their juices into the mixture.

Continue to cook on low heat for an additional 5-7 minutes, stirring occasionally. The filling should become syrupy and glossy. Remove from heat and stir in the vanilla extract for added flavor.

Allow the strawberry pie filling to cool to room temperature before using it in your pie or as a topping. This cooling step is important as it helps the filling set up properly.