Preheat and Prep: First, preheat your oven to 180°C (160°C fan). Lightly grease a 2.5-litre (or 9x13 inch) baking dish and set it aside.

Cook the Chicken: Pat the chicken cubes dry with a paper towel and season generously with salt and pepper. Heat the olive oil in a large, oven-safe skillet or frying pan over medium-high heat. Add the chicken in a single layer (work in batches if needed) and cook for 5-7 minutes, turning occasionally, until golden brown on all sides. The chicken doesn't need to be cooked through at this stage. I find that getting a good sear on the chicken is crucial for flavour. Transfer the browned chicken to your prepared baking dish.

Sauté the Aromatics: In the same skillet, reduce the heat to medium. Add the chopped onion and cook for 3-4 minutes until it softens and becomes translucent. Add the minced garlic and cook for another minute until fragrant, being careful not to let it burn.

Wilt the Spinach: Add the fresh spinach to the skillet in handfuls. It will look like a lot, but it wilts down significantly. Stir continuously for 2-3 minutes until all the spinach has wilted. Drain off any excess liquid that has been released.

Create the Creamy Sauce: Reduce the heat to low. Add the softened cream cheese, double cream, and grated Parmesan to the skillet. Stir constantly until the cream cheese has melted and the sauce is smooth and combined. Stir in the dried oregano, nutmeg, and season with salt and pepper to your liking.

Combine and Bake: Pour the creamy spinach sauce evenly over the chicken in the baking dish. Gently stir to ensure all the chicken is coated.

Add the Cheese Topping: In a small bowl, mix the grated mozzarella and cheddar cheese. Sprinkle this mixture evenly over the top of the casserole.

Bake to Perfection: Place the dish in the preheated oven and bake for 20-25 minutes, or until the sauce is bubbling and the cheese on top is melted and lightly golden. Let it rest for 5 minutes before serving.