Add the berries and sugar to your sauce pan and cook them over medium heat, until the sugar melts and the berries soften down. Smash up the berries with your spatula to release the juices and stir it up. Cook this for around 5 minutes.
1 Pint Raspberries, ¼ Cup White Sugar
Pour the raspberries into your strainer over some vessel like a bowl or glass measuring cup. Push and work at the pulpy mass to get as much juice through the strainer as you can.
Transfer the strained juice back to the sauce pan. Cook briefly over medium heat to re-warm and loosen it up.
Mix the cornstarch and water into a slurry, then add it to the raspberries. Stir until it thickens.
½ Tsp Corn Starch, 1 Tsp Water
Remove the coulis from the heat and pour it into a bowl or jar to cool. Use immediately, store in the fridge for up to a week, or freeze for later use.