Break down the raspberries by adding them to a saucepan along with the sugar, water, and lemon juice. Stir occasionally until broken down. This will take about 5 minutes.
Add the mixture to a blender and puree.
Pour the blended mixture through a strainer and discard the raspberry seeds.
Let the mixture cool to room temperature, and then store it in an airtight container in the fridge.
Notes
Store the leftover raspberry coulis in an air-tight container in the refrigerator for 4-5 days.