Rub the chicken with thyme, salt and pepper, then leave for 24 hours in a refrigerator. Roast the chicken in a roasting bag in an oven set at 73°C for 30 minutes, then raise the temperature to 150°C for 10 minutes until the skin is golden.
Peel the vegetables, then boil and season with salt and pepper.
Soak the pearl barley and cook it, fry the onions, garlic and mushrooms in a pan, add the barley and cook while stirring, season with salt and pepper.
Serve with pearl barley and vegetables, decorated with yarrow leaves.