Put raspberries, sugar and water into a microwaveable bowl. microwave for 2 minutes. stir. If using fresh berries, 2 minutes could be enough. What you want to do is soften the berries. If the mixture starts to foam up, that is enough. The frozen berries took another minute in the microwave to start foaming. So three minutes total.

Add the lemon juice and place sauce into a blender or food processor. Pulse the blender three or four times. You want to break up the raspberries, but not so much that the seeds are getting chopped up too much.

Use a fine strainer to strain the sauce. Use a spatula to swirl the sauce and force it through the strainer. This can take several minutes. Keep working it until most of the moisture has been saved.

Put the coulis in a squirt bottle and store in the refrigerator for up to a couple weeks.