1. Add the raspberries, sugar and lemon juice to a large saucepan and cook over medium heat.
2. When raspberries begin to release their juices, smash them with your spoon or spatula.
3. Let sauce continue to cook and boil for about 10-15 minutes over medium heat. The time required to thicken the sauce can vary based on your particular stove. Keep in mind that it will thicken more as it cools. The longer you cook it, the thicker it will get. If it ends up a little too thick, you can always add a touch of water to thin it back out.
4. When sauce has thickened to your liking, remove it from heat and pour into a food processor. Puree the sauce until it is smooth.
5. Strain the sauce through a fine mesh sieve, and then allow to cool.