Preheat the oven and an appropriately sized roasting tray to 425 degrees F. Wash your chicken inside and out and pat dry with kitchen paper. Using your fingers, part the breast skin from the breast meat. It's important to try to push your hand gently down the breast, being careful not to rip the skin. With a peeler, remove and chop the fragrant yellow skin of the lemon, keeping the peeled lemon to one side. Chop prosciutto and add to the bowl with the lemon skin, garlic and thyme. Season, with salt and pepper and then mix it all into the butter. Push this into the space you have made between the meat and the skin -- rub and massage any that's left over in and around the bird. It's all tasty stuff. I could tell you to tie the chicken up but I've decided it's a palaver and not worth it in this case. Slash the thigh meat to allow the heat to penetrate a little more, which makes it taste better. Cut the peeled lemon in half and push it into the cavity. Then put chicken in the hot roasting tray and roast in the preheated oven for 20 minutes.
While the chicken is cooking, parboil the potatoes in salted water for 10 minutes and drain. Cut the celeriac into irregular chunks around the same size as the potatoes. Remove the chicken from the oven, by which time the tasty butter will have melted, flavored and cooked out of the chicken into the bottom of the tray, awaiting your potatoes and celeriac. Normally I put a fork into the cavity of the chicken and lift it off of the tray for 20 seconds while I toss and coat the vegetables in the butter. Put the chicken back on top of the vegetables and cook for around 40 minutes. Leave to stand for 10 minutes. Once the meat and vegetables have been removed, a little light gravy can be made in the tray with a splash of wine and stock, a little simmering and scraping.