Prepare strawberries: Wash, hull, and slice strawberries. Set aside.
Make the base: In a saucepan, whisk together sugar, cornstarch, and water until smooth.
Cook the mixture: Heat over medium heat, stirring constantly until it becomes thick and glossy.
Add strawberries: Stir in strawberries and cook for 3–5 minutes until softened but still slightly chunky.
Finish the filling: Remove from heat, add lemon juice and vanilla, and mix well.
Cool completely: Let the strawberry pie filling recipe cool before using. It will thicken more as it rests.