Bring about 8 oz of the raspberries, jam, sugar, and 1 tsp of the lemon juice to a boil in a medium saucepan.
Reduce heat and simmer 2-3 minutes, or until raspberries begin to break down.
Stir in cornstarch/water mixture and return to a boil.
Remove from heat and let cool. If desired, mash some of the whole raspberries, and strain seeds.