Add the frozen strawberries, water, and sugar to a pot or high-sided sauce pan. Put the pan on medium heat and stir occasionally, letting the strawberries become soft, this will take about 3-4 minutes.
Pour this mixture into a small blender and blend until smooth - be careful that the mixture is not too hot when you blend it (this will build up pressure in the blender), so let it cool for a few minutes before blending if needed.
Put the mixture back in the pan and add the lemon juice. Continue to cook on medium heat, stirring occasionally, for about 10-15 minutes or until the jam has thickened. You want it smooth and thick enough so that when you run a spatula across the bottom of the pan, you can see the bottom of the pan for a few seconds before the jam covers it again. Remove this from the heat and set aside.