Roux - this one is equal amounts of flour and butter cooked until a medium brown. This will thicken the soup.
Once I browned the aromatics in oil and added mushrooms for a few minutes, I take that mix and put in the crock pot. I add water or broth, sherry (I use about a half cup), fresh herbs, cooked roux. I add the sherry at the end of cooking time and season appropriately with ground pepper and salt.
I used baby portabello mushrooms here, but sometimes I mix a few kinds of mushrooms for more dynamics.