Add the strawberries, sugar and salt to a small saucepan. Mix the cornstarch with 1/2 cup water in a small bowl and add it to the saucepan. Slowly heat the mixture over medium heat, stirring constantly, until it comes to a simmer, about 5 minutes. Lower the heat so it's at a very gentle simmer and cook, stirring frequently, until the sauce thickens, about 20 minutes. Remove from the heat, stir in the lemon zest and juice and let cool.