In a blender or food processor, puree berries until smooth. Strain puree through a fine-mesh strainer into a bowl, pressing with a flexible spatula to extract as much juice as possible from the seeds and pulp. Whisk in lemon juice and sugar to taste (4 TBS of sugar is listed, but add up to 2 more TBS if you wish, depending on your taste preference). Sauce can be covered and refrigerated for up to 1 week.