Heat butter in a large skillet set over medium heat. Cook onion, carrots and celery for 3 to 5 minutes. Remove from heat and add flour. Stir and cook on low heat for 3 to 5 minutes.
Heat butter in a large skillet set over medium heat. Cook onion, carrots and celery for 3 to 5 minutes. Remove from heat and add flour. Stir and cook on low heat for 3 to 5 minutes.