Preheat your oven to 375°F (190°C) and lightly grease a 9×13-inch baking dish.
In a large skillet, heat olive oil or butter over medium-high heat. Sear chicken on both sides (2–3 minutes per side) until golden. Set aside.
In the same skillet, add onion and garlic. Sauté for 1–2 minutes until fragrant.
Add mushrooms and cook until browned and moisture has mostly evaporated, about 5–6 minutes.
Stir in spinach and cook just until wilted.
Pour in the heavy cream and stir to combine. Simmer for 1–2 minutes. If desired, stir in a handful of shredded cheese to thicken.
Transfer the mushroom-spinach mixture to the baking dish. Arrange seared chicken over the top. Pour cream sauce evenly over the chicken.
Sprinkle with remaining mozzarella and optional Parmesan cheese.
Bake uncovered for 25–30 minutes or until bubbly and chicken reaches 165°F (74°C).
Let rest for 5 minutes before serving. Garnish with herbs or extra cheese, if desired.
