Start by laying the chicken breasts between two sheets of clingfilm. Using a rolling pin, gently pound them until they are about 1cm thick. This helps the meat cook faster and more evenly. Season both sides with salt and black pepper.
Heat 1 tablespoon of olive oil in a large frying pan over medium-high heat. Once the oil is hot, place the chicken in the pan and sear for 5 minutes on each side until golden brown and cooked through. Remove the chicken and set it aside on a plate to rest while you prepare the sauce.
Add the remaining tablespoon of olive oil to the same pan. Toss in the sliced mushrooms and sauté for about 4–5 minutes until they are soft and browned. Stir occasionally to prevent them from burning. Remove the mushrooms and keep them aside with the chicken.
Now, deglaze the pan by pouring in the red wine vinegar. Let it bubble for a minute or two until it reduces by half. This removes all the flavorful bits stuck at the bottom and adds a nice tang to the dish.
Pour in the chicken stock and allow it to simmer for 5–10 minutes until slightly reduced. This step concentrates the flavor. Once reduced, stir in the double cream. Mix well until everything is combined and creamy.
Start adding freshly ground black pepper—go with a generous pinch, taste, and adjust to your liking. The bold, spicy flavor should come through but not overpower. Return the mushrooms and chicken (with any resting juices) to the pan. Let everything simmer together for a few minutes until hot and coated in the sauce.
Serve the peppercorn chicken hot with buttery mashed potatoes and steamed greens like broccoli or green beans. The creamy sauce pairs perfectly with the sides and soaks into the mash beautifully.