Preheat oven to 375°F (190°C) and grease a baking dish with olive oil.
In a skillet, heat olive oil over medium heat and sauté onions and garlic until fragrant, about 2 minutes.
Add diced chicken, mushrooms, Italian seasoning, salt, and pepper; cook until chicken is browned and cooked through, about 5-7 minutes.
Stir in spinach and heavy cream, then mix in mozzarella and Parmesan cheeses until combined. Pour into the prepared baking dish.
Bake for 20-25 minutes, until bubbly and golden on top.
