Preheat your oven to 375°F (190°C).
In a skillet, heat olive oil over medium heat. Add minced garlic and sauté for about 1 minute until fragrant.
Add sliced mushrooms to the skillet and cook for about 5 minutes until they are tender. Stir in the fresh spinach and cook until wilted, about 2-3 minutes. Remove from heat.
In a mixing bowl, combine heavy cream, dried oregano, salt, and pepper. Mix well to combine.
Place the chicken breasts in the baking dish. Pour the cream mixture over the chicken, followed by the cooked spinach and mushroom mixture. Spread it evenly over the top.
Sprinkle shredded mozzarella cheese over the entire dish.
Cover the baking dish with aluminum foil and bake in the preheated oven for 20 minutes.
After 20 minutes, remove the foil and bake for an additional 10 minutes until the cheese is bubbly and golden.
Once done, let it cool for a few minutes before serving.