Preheat and Prepare: First, preheat your oven to 180°C (160°C fan). Have a large ovenproof casserole dish (around 2-litre capacity) ready.

Brown the Chicken: Heat the olive oil and 1 tbsp of butter in a large, heavy-based pan or skillet over a medium-high heat. Season the chicken pieces with salt and pepper. Working in two batches to avoid overcrowding the pan, brown the chicken on all sides for 5-7 minutes until golden. It doesn’t need to be cooked through. Remove with a slotted spoon and set aside.

Sauté the Aromatics: Lower the heat to medium. Add the chopped onion to the same pan and cook for about 5 minutes until softened. Add the minced garlic and sliced mushrooms and cook for another 5-7 minutes, until the mushrooms have released their liquid and started to brown.

Create the Sauce Base: Add the 50g of butter to the pan. Once melted, stir in the plain flour and cook for 1-2 minutes, stirring constantly. This is your roux, and it will thicken the sauce.

Build the Creamy Sauce: Gradually pour in the warm chicken stock, a little at a time, whisking continuously to prevent lumps. Once all the stock is incorporated and the sauce is smooth, bring it to a gentle simmer. Let it bubble for 2-3 minutes to thicken slightly.

Combine and Season: Stir in the double cream and fresh thyme leaves. Season generously with salt and pepper to taste. Return the browned chicken pieces (and any juices) to the pan and stir everything together. Let it simmer gently for another minute. What works best for me is tasting the sauce at this stage to ensure the seasoning is just right before it goes into the oven.

Assemble the Casserole: Carefully pour the chicken mixture into your prepared casserole dish and spread it out evenly.

Prepare the Topping: In a small bowl, mix the breadcrumbs, grated cheddar, and grated Parmesan. Sprinkle this mixture evenly over the top of the chicken filling.

Bake to Perfection: Place the dish in the preheated oven and bake for 25-30 minutes, or until the topping is deep golden brown and the sauce is bubbling up around the edges.

Rest Before Serving: Remove from the oven and let the casserole stand for 5-10 minutes before serving. This allows the sauce to thicken slightly and makes it easier to serve.