1) In a saucepan, place the celery and 1 bouillon cube. Cover these with enough water.
2) Place the saucepan over moderate heat to simmer, uncovered, until celery is soft and most of the water is evaporated.
3) In a blender, place the mixture to process to a smooth mixture. Alternately, press the soft celery through a sieve.
4) Return the mixture to the saucepan.
5) Add the second bouillon cube by crumbling along with onion and mushrooms.
6) Frequently stir the mixture to simmer for about 10 minutes.
7) Serve the sauce with your choice of main dish or side dish.