Divide the strawberries in half and place one half to the side. Puree half of the strawberries with the lemon juice in a blender or a food processor.
In a medium saucepan, combine the strawberry puree, sugar, corn starch, vanilla extract, and a pinch of salt. The exact amount of sugar will depend on how sweet your strawberries are. If the berries are very ripe and sweet, ¾ cup is usually enough. If they are a little tart, increase the sugar to about 1 cup. Cook over medium heat. Stir the strawberry mixture constantly until it thickens and comes to a boil. This will take about 5-7 minutes.
Once thickened, remove the saucepan from the heat, pour the mixture through a sifter to remove any cornstarch clumps, and allow it to cool slightly.
In a large bowl, gently fold the reminder of the fresh strawberries into the cooled mixture. Make sure the strawberries are coated evenly with the strawberry sauce.