Gut the boiling fowl and rinse (inside and out) in cold water, pat dry, place in a large pan. Place the onions (cut side down) in a frying pan, cook over a medium heat for approx. 10 mins. until dark brown. Add the onion to the chicken along with all the other ingredients up to and including the salt, cover with cold water (approx. 4 l) and gradually bring to just below the boil. Cook (uncovered) at just below the boil for approx. 1½ hrs., skim away the froth with a sieve.
Remove the chicken from the stock, allow to cool slightly. Strain the stock, pour approx. 1½ l into a pan, set aside the remainder of the stock. Boil fast for approx. 10 mins. Pull apart the chicken meat by hand, add to the stock with the vegetables and pasta, simmer for approx. 10 mins. until the pasta is al dente.
Mix the parsley, lemon zest, lemon juice, oil, garlic and fleur de sel. Serve the soup in deep bowls, top with the gremolata.