saute (in a butter/ oil mix) between 2 and 4 large onions until soft and starting to brown. add 2 celery stalks, chopped. add 2 baskets (minimum - וכל המוסיף...) of mushrooms, sliced (I like to use some baby portabellas and some regular)and continue to cook until most of the liquid is cooked out. mix in 2-3 tablespoons of flour, some salt, a half teaspoon of nutmeg, and 2 teaspoons of granulated garlic. Next comes about a liter of milk (or use heavy cream for some of it - about a cup), gradually. Continue cooking on a low flame, adding up to another liter of milk / water, until it is heated through.
I occasionally put in canned mushrooms for part of it, and if the flavor isn't "mushroomy" enough, you can also put in a bit of powdered mushroom soup or sauce mix, but it is so much better without, in my opinion! Enjoy!