Preheat and prep: Heat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish.

Sauté the mushrooms: Heat butter and olive oil in a large skillet over medium-high. Add sliced mushrooms and a pinch of salt. Cook 6–8 minutes, stirring occasionally, until golden and most liquid has evaporated.

Add aromatics: Reduce heat to medium. Stir in the onion and cook 3–4 minutes until softened. Add the garlic and cook 30 seconds, just until fragrant.

Build the sauce: Pour in the chicken broth and scrape up any browned bits from the pan. Stir in the heavy cream and bring to a gentle simmer.

Melt in the cheeses: Add cream cheese in small chunks, stirring until smooth. Mix in the Parmesan, thyme, smoked paprika, and a few grinds of black pepper. Simmer 2–3 minutes to slightly thicken. Taste and adjust salt.

Add greens (optional): Stir in the chopped spinach and cook 1–2 minutes until wilted.

Combine with chicken: Fold the cooked chicken into the sauce until evenly coated.

Assemble: Transfer the mixture to the prepared baking dish. Spread evenly and top with shredded mozzarella.

Bake: Bake 18–22 minutes, until the casserole is bubbling and the cheese is melted and lightly golden.

Rest and serve: Let it rest 5–10 minutes before serving so the sauce sets slightly. Sprinkle with extra thyme or chopped parsley if you like.