For marinade: Prepare the marinade evening before you are going to roast the chicken. Combine in a bowl salt, sugar, pepper, garlic cloves and onion cut into chunks. Pour in some hot water to help it melt then add as much cold water as to immerse the whole chicken. Cover with aluminium foil and let soak overnight in the fridge. Next morning take the chicken out of the marinade dry it well and leave to rest.
For aromatic butter: Combine in a bowl softened butter, grated garlic, grated lemon zest, finely chopped rosemary and parsley, salt and pepper, mix well. Coat the chicken with aromatic butter outside and inside.
Roasting: Peel potatoes cut in half and cook in plenty of salted water for about 15 minutes, drain and distribute over roasting pan bottom. Between potatoes distribute small red onions.
Place the chicken on top. You may stuff chicken with lemon quarters. Tie chicken legs with rope. Cover the roaster and put into oven to bake for one hour at 180°C. After that, from time to time open the roaster, cover the chicken with some aromatic butter and continue roasting. After two hours take off the cover to add garlics bulbs cut in half, rosemary branch and champignons then bake open at 220 °C. From time to time you may coat the chicken with some aromatic butter.
When chicken skin is crunchy brown take it out of the roaster together with garlic and rosemary and place in the roaster lid, then cover with aluminium foil. Extract most of the liquid from the roaster into a pan and return the roaster into oven to brown the vegetables.
You may make a gravy from the roasting liquid: bring it to boil, add the rest of aromatic butter with 1-2 tablespoons of flour mixing with wire all the time to break all lumps. When gravy is thick enough pour it into serving bowl.
Serve the chicken with gravy and vegetables on the side, garlic bulb halfs and slices of lemon.