Heat the oven to 400°F (200°C). Lightly grease a 9×13-inch baking dish.
Pat chicken dry. Season both sides with 1/2 teaspoon salt, pepper, paprika, and half the Italian seasoning. Drizzle with 1 tablespoon olive oil. Place in the baking dish.
In a large skillet over medium heat, warm 1 tablespoon olive oil. Sauté mushrooms with a pinch of salt for 4–5 minutes until they release liquid and brown.
Add garlic. Cook 30 seconds until fragrant. Stir in spinach and toss just until wilted, 1–2 minutes. Remove from heat.
In a bowl, whisk cream cheese with chicken broth (or use heavy cream). Add remaining Italian seasoning, parmesan, and red pepper flakes if using. Stir in the mushroom-spinach mix. Taste and adjust salt.
Spoon the creamy mixture over the chicken. Top with mozzarella. Bake 20–25 minutes, until the chicken reaches 165°F (74°C) at the thickest point and the top is bubbly.
Rest for 5 minutes. Add lemon zest if using. Spoon sauce over each piece. Serve hot with salad, cauliflower rice, or zucchini noodles.
Notes
For best flavor, brown the mushrooms well and let the dish rest before serving.