Presoak a clay pot, top and bottom, in water for 15 minutes.
Saute the celery, mushrooms, and onion in the butter and olive oil. Add the garlic, salt, mint, oregano, pepper and lemon juice.
Cook the rice for half the time required by the directions on the package, then combine, cooking liquid and all, with the celery-mushroom mixture.
Mix well, then stuff this mixture in