Prep your oven and pan: Preheat the oven to 375°F (190°C). Grease a 9x13-inch casserole dish with butter or a light coat of olive oil.

Cook the veggies: Heat olive oil and 1 tablespoon butter in a large skillet over medium heat. Add onion and mushrooms with a pinch of salt and cook 5–6 minutes until soft and lightly browned.

Add aromatics: Stir in garlic and cook 30 seconds until fragrant. Add broccoli and cook 3–4 minutes to soften slightly. Toss in spinach and cook just until wilted. Transfer the mixture to a large bowl.

Make the creamy base: In the same skillet over low heat, add the remaining 1 tablespoon butter and cream cheese. Stir until the cream cheese softens and becomes smooth.

Build the sauce: Whisk in heavy cream and chicken broth slowly until the sauce is silky and cohesive. Season with smoked paprika, thyme, ground mustard, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Simmer 2–3 minutes to thicken slightly.

Combine the filling: Add the cooked chicken to the bowl with the veggies. Pour the creamy sauce over everything, then add 1 cup shredded cheese. Stir gently to coat and combine.

Assemble: Spread the mixture evenly in the prepared casserole dish. Top with the remaining 1 cup shredded cheese and the grated Parmesan. Sprinkle with a pinch of red pepper flakes if you like heat.

Bake: Bake for 18–22 minutes, until the edges bubble and the top turns golden in spots. For deeper browning, broil for 1–2 minutes at the end—watch closely.

Rest and serve: Let the casserole sit for 5–10 minutes to set. Taste and adjust salt and pepper if needed. Serve with a crisp green salad or roasted asparagus.