1. Step
Heat oil in a Dutch oven or large saucepan over medium-high heat. Add onion, carrot and celery; cook, stirring often, until the onion and celery soften, 5 to 6 minutes. Add potato and garlic; cook, stirring, for 2 minutes. Stir in flour, dry mustard and cayenne; cook, stirring often, for 2 minutes.