In a blender or food processor, blend the raspberries and sugar until well combined and as smooth as possible.
Strain the mixture through a sieve into a bowl to remove all the seeds. You'll need to press and rub with a rubber spatula until only the seeds are left in the strainer. (This is a little tedious, but worth it for a nice smooth, seedless texture.)
Stir in the Grand Marnier, If desired.
Makes 1 cup, which can be covered and stored in the refrigerator up to a day ahead.