Combine half the strawberries, sugar, lemon juice, cornstarch, and vanilla in a small saucepan over medium-low heat. Cook so the strawberries begin to break down and the sauce thickens, about 10 minutes. Break them up with the back of your spoon.
1 1/2 pounds fresh strawberries, 1/2 cup granulated sugar, 2 teaspoons lemon juice, 1 1/2 teaspoons cornstarch, 1 teaspoon vanilla extract