Preheat the oven to 350 degrees F. Butter a 9 by 13-inch casserole.
Steam the broccoli until tender, about 10 minutes. In the meantime, melt the butter in a medium skillet over medium heat and saute the celery, mushrooms, and onion until softened, about 10 minutes; drain. Combine the broccoli and the cooked vegetables in a bowl.
Heat the soup and softened cheese product in a saucepan over low heat until the cheese melts. Pour it over the broccoli mixture. Add the garlic salt and pepper and combine.
Put it into the buttered casserole dish and bake for 20 to 25 minutes. Sprinkle the top with the grated Cheddar the last 5 minutes of baking.