Preheat oven to 400° F. Spray an 8- or 9-inch square baking dish with cooking spray and set aside.
Cook broccoli in microwave, just until crisp tender (about half of the time called for on the package). You don’t want to cook the broccoli until it’s soft because it will continue cooking when you bake the casserole. Place broccoli in colander to drain.
In a small saucepan over low heat, stir together condensed soup, cheese, and lemon juice, just until cheese melts.
In a separate bowl, whisk together egg, mayonnaise, and grated onion. Combine broccoli, soup mixture, and egg mixture. Transfer to prepared baking dish.
In a small bowl, stir together dry stuffing mix and melted butter. Sprinkle on top of the casserole.
Bake for 15-20 minutes, or until heated through and topping is golden brown.