How can I use self made cream of celery as a replacement for mushrooms in green bean casserole? I follow Alton Brown's Green Bean Casserole for the most part and have had great success making this recipe. My dad always complains about having mushrooms in the dish and he would have to pick them out. But this year I would like to try making without mushrooms.

My goal is to use another vegetable to replace the mushrooms when making the creamy sauce. From what I have heard you can use cream of celery soup but I would still like to make the cream myself. Can I use cut up celery as a replacement in Alton Brown's recipe? Do I need to alter cooking times? I think I would use 1 cup of chopped up celery.
After reading your original question, the revisions, and the comments, I have what may be an option for you. As noted in comments, you can use just about any vegetable you want. 

To keep the flavor profile the same, make the creamy sauce as you usually do and then strain it well through a colander to remove the mushrooms. (You can also do this with celery.) By doing this you will have the flavor of mushrooms, celery, or whatever vegetable you choose in your dish.

The other option would be to purée your sauce.

Then, if you want a vegetable (of your choice) to add for texture, sautée them in a very small amount of butter to your desired consistency and add to your casserole.

If you don't feel the need for adding another vegetable, simply don't add any.