Prepare the Oven and Chicken: Preheat your oven to 200°C (180°C Fan/Gas Mark 6). Pat the chicken chunks dry with a paper towel and season generously with salt and pepper.

Sear the Chicken: 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) and cook for 2-3 minutes per side, until golden brown. The chicken doesn't need to be cooked through at this stage. Remove the chicken from the pan with a slotted spoon and set it aside.

Sauté the Aromatics: Reduce the heat to medium. Add the chopped onion to the same pan and cook for 5-7 minutes, until softened and translucent. Add the sliced mushrooms and cook for another 5 minutes until they've released their liquid and started to brown. Stir in the minced garlic and cook for one minute more until fragrant.

Create the Creamy Sauce: If using, pour the white wine into the pan to deglaze, scraping up any browned bits from the bottom. Let it bubble and reduce by about half. Stir in the chicken stock and Dijon mustard, bringing the mixture to a simmer.

Bring It All Together: Reduce the heat to low and slowly stir in the double cream. Return the seared chicken and any accumulated juices to the pan. Add the fresh spinach in handfuls, stirring until it wilts into the sauce.

Bake to Perfection: Season the sauce with salt and pepper to your liking. Stir in half of the grated Parmesan cheese and the chopped parsley. If your pan isn't oven-safe, transfer everything to a 9x13 inch (or similar sized) baking dish. Sprinkle the remaining Parmesan over the top.

Final Bake: Place the casserole in the preheated oven and bake, uncovered, for 15-20 minutes, or until the sauce is bubbling and the top is lightly golden. Let it rest for 5 minutes before serving.