Precook 1 cup raw rice (not minute rice). Drain well. Put into a casserole dish. Mix in 6 1/2 ounces drained tuna. Saute 1/2 medium onion and 2 stalks celery in butter until barely tender. Add to tuna and rice. Mix 1 can cream of mushroom soup with 1 can water and heat. Pour over tuna and rice mixture. Cook in 350 degree oven for about 35 minutes. Rice should absorb soup and be tender. If needed add a little water and continue cooking until rice is tender. Season to taste with salt and pepper.