Add the raspberries and sugar into a small saucepan. Put the saucepan over low-medium heat and cook for a few minutes.
In a small bowl mix lemon juice and cornstarch together to create a slurry, this will thicken the coulis. Then pour the slurry into the saucepan.
Mix everything together and using a masher help break raspberries up.
After 5 minutes of cooking over medium-low heat take the pot off of the heat and pour into a blender or food processor.
Place a fine-mesh strainer over a large clean bowl, pour the sauce through the strainer, use a spatula to move the seeds around and help the liquid push through.
Pour sauce into an air-tight container and store in the fridge for a few hours before using.