In a bowl, roughly mash your strawberries with a potato masher (you may need to lean into it at first, to get them going) or squeeze them with your fingers. Put them into a large skillet (cast iron is perfect!) with the sugar and lemon juice and cook over medium-high heat, stirring often and breaking up large chunks of berry with your spoon, until it thickens and your spoon leaves a trail across the bottom of the pan. (It should take about 10 minutes.) If you like, stir in the vanilla. Cool. Makes about 1 1/2 cups.