Remove the chicken from the fridge 30 minutes prior to cooking. Set the oven to 410°F/210°C
Make the compound butter by mixing everything together in a bowl with a spoon or whisk.
Peel and wash the potatoes. Slice the big ones in half and keep the small ones whole. Cook in a small pot until fork tender (about 10 minutes).
Once the potatoes are cooked, strain them and try to remove as much water as possible. Toss with a little oil and salt and line up in one even layer in the roasting dish. Allow to cool before adding the chicken.
Remove the wishbone first. Then flip the chicken on its front and cut along the back as shown.
Cut along the other side until you remove the back bone completely (save this piece for the gravy).
Flip the chicken over and cut the breast bone with the scissors to help the chicken lie flat.
Use your fingers to gently separate the skin from the breast meat. Insert all the compound butter underneath the skin.
Cover the skin side of the chicken with regular softened butter and season heavily with salt (if brined, then season lightly).
Flip the chicken over and season with garlic powder and salt.
Place the seasoned chicken skin side up over the cooked potatoes and roast in the oven for 40-45 minutes or until thighs reach 165°F/74°C on a thermometer.
Meanwhile start with the gravy. Begin by browning the reserved chicken back in a little oil in a small pan.
Slice the onions and garlic finely.
Once the chicken back is browned nicely, remove the oil from the pan and discard. Add the onions, garlic and cook for 1-2 minutes without browning. Deglaze the pan with a shot of Cognac (optional) and then add the chicken stock, dried porcini and herbs.Bring to a boil, then simmer until the chicken is ready. Once the chicken is ready, carefully remove it from the potatoes onto another plate (a big spatula works well here). Carefully drain off the fat from the roasting dish and save 40g worth of fat.Turn the oven up to 425°F/210°C and put the potatoes back in the oven to crisp up while you make the gravy. Set a timer for 15 minutes.
Get a large saucepan hot and add the butter. Once its foamy, add the mushrooms and cook until nicely browned. Season with salt and pepper and reserve in a bowl.
Measure out 40g of the reserved chicken fat (or just use butter) and heat in the same pan as the mushrooms. Add the flour and whisk until smooth. Cook for 3-5 minutes on low heat until light brown (blonde roux). Let the roux cool for 5 minutes. Strain the fortified stock and measure out 500ml (add more chicken stock if you don't have enough). Slowly pour this stock into the roux while whisking vigorously to remove any lumps.Add the rest of the stock and bring to a boil.
Finish the gravy with the mustard, the sauteed mushrooms and season with salt and pepper.
To Serve:Remove the potatoes from the casserole dish and drain on paper towels. Place the chicken back in the oven for 5 minutes to reheat it and bring the gravy to a boil. Carve the chicken at the table or slice it into pieces as shown. Enjoy!