Prepare the Chicken: Pat the chicken breasts dry with a paper towel. This helps them get a beautiful golden-brown sear. Season both sides generously with the salt and pepper.

Sear the Chicken: Heat the olive oil in a large, heavy-based frying pan over a medium-high heat. Place the chicken breasts in the pan, ensuring not to overcrowd it (cook in two batches if necessary). Cook for 5-6 minutes on each side until golden brown and cooked through. The internal temperature should reach 74°C. Remove the chicken from the pan and set it aside on a plate.

Sauté the Onions: Reduce the heat to medium-low. Add the butter to the same pan. Once it's melted, add the sliced onions. Cook for 8-10 minutes, stirring occasionally, until they are very soft, sweet, and lightly caramelised.

Cook the Mushrooms and Garlic: Add the sliced mushrooms to the pan and increase the heat to medium. Cook for 5-7 minutes until they have released their liquid and started to brown. Add the minced garlic and dried thyme and cook for another minute until fragrant.

Create the Sauce Base: Sprinkle the plain flour over the vegetables. Stir continuously for one minute to cook out the raw flour taste. This mixture, known as a roux, will thicken our sauce.

Build the Sauce: Gradually pour in the chicken stock, stirring or whisking constantly to avoid lumps. Scrape the bottom of the pan to lift any browned bits – this is where so much flavour is. What works best for me is adding the stock in three or four parts, making sure each addition is fully incorporated before adding the next. Bring the sauce to a gentle simmer and let it thicken for 2-3 minutes.

Make it Creamy: Stir in the double cream and Dijon mustard. Let it bubble gently for another minute, then taste and adjust the seasoning if needed.

Smother the Chicken: Return the seared chicken breasts (and any juices from the plate) to the pan, nestling them into the sauce. Spoon some of the sauce and vegetables over the top of each breast.

Melt the Cheese: Sprinkle the grated Gruyère cheese evenly over the chicken. You can either place a lid on the pan for 2-3 minutes to let the cheese melt, or for a golden, bubbly top, place the entire (oven-safe) pan under a hot grill for 2-4 minutes until the cheese is melted and bubbling.

Garnish and Serve: Sprinkle with fresh chopped parsley and serve immediately.