1. Use a food processor to finely dice/puree the onion, carrot and celery (you may have to do these separately depending on the size of your food processor).
6. Take out about 1.5 cups of the hot stock out of the crock pot and slowly, 1/4 cup of a time, whisk into the flour in the skillet. Whisk one addition completely in before adding the next 1/4 cup.