Preheat and Prepare: First, preheat your oven to 200°C (180°C fan). Lightly grease a 2-litre (approximately 9x13 inch) baking dish and set it aside.

Sear the Chicken: Season the chicken chunks generously with salt and pepper. Heat the olive oil in a large, oven-safe skillet or frying pan over a medium-high heat. Add the chicken in a single layer (work in batches if necessary to avoid overcrowding) and cook for 3-4 minutes per side, until golden brown. The chicken doesn't need to be cooked through at this stage. Transfer the browned chicken to the prepared baking dish.

Sauté the Aromatics: In the same skillet, reduce the heat to medium. Add the chopped onion and cook for 5-7 minutes, stirring occasionally, until softened and translucent. Add the minced garlic and cook for another minute until fragrant.

Build the Creamy Sauce: Add the softened cream cheese to the skillet, stirring until it melts into the onions and garlic. Slowly pour in the chicken stock and single cream, whisking continuously to create a smooth sauce. Bring it to a gentle simmer.

Wilt the Spinach: Add the fresh spinach to the sauce in large handfuls. It will look like a lot, but it wilts down very quickly. Stir until all the spinach has wilted into the sauce, which should take about 2-3 minutes.

Finish the Sauce: Stir the Dijon mustard, ground nutmeg, and fresh lemon juice into the sauce. Season with salt and pepper to your taste. What works best for me is tasting at this stage to make sure the seasoning is just right before pouring it over the chicken.

Assemble the Casserole: Pour the creamy spinach sauce evenly over the chicken in the baking dish. Gently stir to ensure all the chicken pieces are well-coated.

Add the Cheese Topping: In a small bowl, toss together the grated Cheddar and Gruyère cheeses. 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 topping is melted and golden brown. For food safety, ensure the chicken is cooked through by checking it reaches an internal temperature of 74°C (165°F), as recommended by the Food Standards Agency.

Rest and Garnish: Carefully remove the casserole from the oven and let it rest for 5-10 minutes. This allows the sauce to thicken slightly and makes it easier to serve. Garnish with fresh chopped parsley before serving.