Preheat the oven to 350°F and lightly coat an 8-inch square (2-quart) baking dish with cooking spray. Distribute the broccoli florets, drained tuna, and 1 1/2 cups of the shredded cheese evenly in the prepared dish. In a separate large mixing bowl, combine the Bisquick mix, sour cream, milk, and eggs, whisking until smooth and well incorporated. Gently pour this mixture over the ingredients in the baking dish. Place the dish in the oven and bake for 30 to 40 minutes, or until a knife inserted into the center emerges clean. Sprinkle the remaining 1/2 cup of cheese on top and allow the bake to rest for 5 minutes prior to serving.