Warm one tablespoon of the olive oil in a large heavy pot. Add onion and sauté until soft (do not brown), about 5 minutes. Add the carrots, the celery and then the pancetta and cook until light brown, about 3 minutes. Add rice and cook, stirring until coated, about 1 minute. Add the thyme and 1 cup stock. Stir constantly with a wooden spoon until stock is almost absorbed, about 1 minute.