Combine Raspberries, sugar and water in medium saucepan. Using a potato masher, crush berries and stir mixture until it comes to a slow boil. Continue to slow boil for 7 minutes, stirring occasionally. Remove from heat.
Strain seeds from sauce, and return juice to saucepan. Thicken juice to desired consistency with a cornstarch/water mixture. Add lemon juice. Remove to bowl and cover surface of sauce with plastic wrap. Chill.
If you do not care about having raspberries seeds in your baked goods, the seeds and pulp removed from the sauce can be ground further in a food processor and used to flavor muffins, cakes, etc.