Place celery and soymilk in saucepan; cook on low heat until celery is tender. Drain and reserve soymilk. In another saucepan, melt margarine and sauté onion. Add flour and make roux, blending in the soymilk the celery cooked in to make cream sauce. Season with salt, pepper and nutritional yeast to taste. I like a dash of tabasco to give it a lift. Add celery. You can serve this as is, or place in an ovenproof pan, top with bread crumbs and brown briefly under the broiler.