Sweat the onion, celery, and bell pepper in butter and oil until soft.
Add garlic toward the end so it doesn't burn.
Add flour, and cook, stirring constantly, just until it starts to take on a little color.
Add chicken broth and bring to a boil. (At this point, you can puree with a stick blender if you like a smoother consistency. I like it either way. If you plan to puree, use a yellow bell pepper so the color doesn't go off.).
Add salt, pepper, and crab boil to taste. (I think Zatarain's has the best flavor for this, but be careful, it is potent stuff. If you can't get crab boil in your area, just use a little cayenne pepper.).