Preheat oven to 375°F (190°C). Grease a 13×9-inch baking dish with butter.
Season chicken with salt, lemon pepper, and paprika. Place in dish, flat side down. Bake 15–20 minutes.
Make creamed spinach: In a skillet, heat 1 tbsp oil over medium. Add spinach, cover, and cook 1–2 minutes until wilted. Stir in garlic and half-and-half, bring to a boil, then add mozzarella. Stir until creamy. Season with salt, set aside.
Cook mushrooms: In another skillet, heat 1 tbsp oil over medium-high. Add mushrooms and sauté 1–2 minutes until golden brown.
Assemble: Remove chicken from oven. Top each piece with creamed spinach, mushrooms, and bacon. Place 1 slice Pepper Jack cheese on top.
Bake again for 15–20 minutes, until chicken is fully cooked (165°F internal temperature).
Optional finish: Broil 2–4 minutes until cheese is golden and bubbly.