Blend raspberries in blender until smooth.
Strain through a fine-mesh strainer to remove seeds.
Pour the resulting coulis into a saucepan and add the sugar and lemon juice. Stir and bring to a simmer.
Heat without boiling to dissolve sugar. Remove from heat and allow to cool before pouring into an airtight container.
If you want a smoother coulis, simply reduce it for longer, stirring constantly to prevent it from burning at the bottom of the saucepan.
Keep refrigerated until ready to use.
For longer storage, refer to the instructions in the article.