Broccoli is such a versatile vegetable; it can be sautéed, stir-fried, roasted in the oven, and even steamed. When it comes to adding broccoli to a casserole dish, it really depends on when during the cooking process you would like to add your broccoli. If you steam your broccoli first, this helps ensure that it will retain its' beautiful green color when heated again later with the rest of the dish's ingredients. Steaming also allows for a more even texture throughout your finished casserole, giving a nice bite to each bite. However, if you choose not to steam your broccoli beforehand, the cooking time of your casserole may need adjustment so that it cooks fully without burning or drying out.  Whichever option you choose, make sure all vegetables are cooked before serving your casserole, as that'll be key to determining its success.