Add the chopped onion, diced carrot, and diced celery. Cook, stirring occasionally, until the vegetables are softened, about 5 to 7 minutes.
Stir in the flour to coat the vegetables, cooking and stirring constantly for about 2 minutes.
Slowly pour in the chicken broth while stirring continuously to prevent lumps. Alternatively, you can whisk the broth well with a whisk to ensure there are no flour lumps.