Classic bisques are thickened with rice, but all of my culinary instructors used roux to thicken their bisques. Rice, no matter how finely you puree it, will always be grainy.
A roux is a combination of equal parts, by weight, of flour and fat that are cooked together to form a paste. The cooking eliminates the raw flour taste, and the fat-coated starch particles are prevented from congregating together in lumps when added to liquid — making roux the front runner to thicken anything you want smooth and creamy — like a bisque.
Island Vittles 