Remove skin from chickens and reserve. Combine chickens, onion, celery, garlic, parsley and thyme in a saucepan large enough to fit both birds. Cover with water (about 3 litres), bring to just below the boil over medium heat, skim scum from surface with a slotted spoon, simmer until stock is well flavoured (3 hours). Strain through a fine sieve, discard solids.
In a separate saucepan, heat 1cm olive oil to 180C. Coarsely chop reserved chicken skins and fry until crisp and golden (3-5 minutes). Drain on absorbent paper, season to taste, cool then finely chop and set aside.
Whisk eggs and lemon juice in a bowl. Add a ladleful of hot chicken stock and blend well with a hand-held blender until the mixture is hot, then return it to the saucepan of chicken stock, taking care not to overheat and curdle the eggs.
Meanwhile, in a separate saucepan, cook risoni in boiling salted water until slightly overcooked (5-10 minutes). Add risoni to soup, check seasoning and adjust if necessary, then ladle into serving bowls and top with chicken crackling.
Avgolemono, Greek egg and lemon soup, is one of the first dishes I learnt to cook so I think of it as a bit like my first love. Tantalisingly pure and seductive, it always leaves you wanting more. This soup is best made with old boiler hens – it’s the only dish I can think of that’s ideal for old Isa Browns. They are savoury birds in the truest sense of the word. You can scatter some Grana Padano on top if you can be bothered.