Melt the butter in a saucepan over medium heat.  Add the onion, celery, garlic, salt and pepper.  Cook, stirring occasionally, for about 5 minutes, until the veggies soften.  Add in the flour and stir or whisk to combine well.  Cook, stirring, for about 1-2 minutes, until the mixture is a light golden brown.  Add in the evaporated milk and chicken broth and cook, stirring, until thickened.