Preheat oven to 400ºF.
If you are using fresh broccoli florets, bring 6 cups water with 1 teaspoon salt to a boil in a large saucepan over medium-high heat. Add the fresh broccoli and boil for 2 to 3 minutes. Drain and rinse with cold water. If you are using frozen broccoli, defrost it in a colander in the sink.
Combine the broccoli, rice, onion, cream of celery soup, butter, onion soup mix, and salt and pepper to taste in a 9x13-inch casserole dish. Add the water and mix well. Stir in the shredded cheese and cover the baking dish with foil.
Place in the oven and bake for about 1 hour, removing the foil and stirring once about halfway through. Test to make sure rice is done. If it’s not, cover and return to the oven for another 15 to 20 minutes.