Preheat the oven to 350. Boil the noodles, drain, and lay them in a 9x11 casserole dish or larger.
Saute the onion and celery over medium heat until just translucent, about 8 minutes or so. Add the cream of celery soup, water, and milk, stirring to combine and heated through. Simmer for about 10 minutes. Add pepper and a scant amount of salt to taste.
Stir in the tuna. Pour the tuna mixture over the noodles and stir to combine. Sprinkle frozen peas over the top.
Melt the stick of butter until liquified in the microwave, about 10 seconds. Pour the butter over the bread cubes in a separate bowl, using your hands to coat all the cubes in melted butter as you pour the butter.
Spread the bread cubes over the top of the casserole and press everything down gently to make an even layer. Bake for about 20 to 30 minute in the oven or until the bread cubes are crisp.