Break the garlic cloves away from the bulbs, place in a small saucepan, cover with water and simmer gently with a lid on for 30 minutes. Leave to cool in the water. Drain the garlic cloves but don’t throw the water away.
Heat the oven to 180°C/gas 4. Carefully ease the skin of the chicken away from the breasts. Place a sprig of rosemary and a layer of lemon slices between the skin and the meat on each side. Stuff the remaining rosemary and lemon, along with the garlic cloves, into the chicken. Season.
Drizzle a heavy-based roasting tin with olive oil, place the chicken on it and cook it according to weight: 20 minutes per 500g and 20 minutes extra. Transfer the cooked chicken to a serving board and keep it warm while you make the sauce. Place the roasting tin directly over a low heat, add the flour and blend it in well. Slowly pour in the reserved garlic water, stirring constantly. Add most of the white wine, bring to the boil and adjust the consistency with extra wine if desired. Pass through a sieve into a saucepan, add the parsley and bring to a simmer.
Carve the chicken and serve with the sauce.