Prep the oven and pan. Heat the oven to 400°F (200°C). Grease a 9x13-inch baking dish or similar casserole pan.
Par-cook the broccoli. Toss broccoli florets with a drizzle of oil, a pinch of salt, and pepper.
Spread on a sheet pan and roast for 10 minutes until bright green and slightly tender. This keeps the bake from getting watery.
Make the buffalo sauce. In a large bowl, whisk the hot sauce, softened cream cheese, Greek yogurt (or sour cream), ranch (or blue cheese dressing), melted butter, garlic powder, onion powder, and smoked paprika until smooth. Taste and adjust salt and pepper.
Combine with chicken. Stir the cooked chicken into the sauce until well coated.
Layer the casserole. Add the roasted broccoli to the baking dish.
Spoon the buffalo chicken mixture over the top and gently mix to distribute.
Add the cheese. Sprinkle mozzarella and cheddar evenly over the casserole.
Bake. Place in the oven and bake for 18–22 minutes, until the edges are bubbling and the cheese is melted and lightly golden.
Finish and rest. Let the bake rest for 5 minutes. Top with sliced green onions and blue cheese crumbles if you like.
Serve. Scoop into bowls and serve with extra ranch or blue cheese on the side. A squeeze of lemon can brighten the whole dish.