Melt the butter in a large pot over medium heat. Add the onion, carrots and celery with a pinch of salt and cook, stirring, until the onion softens, about 7 minutes. Add the garlic and cook 1 minute more.
Pour in the stock and water, then add the chicken thighs, bay leaves, thyme and salt. Bring to a boil, then lower the heat and simmer, partly covered, for 30 minutes, skimming off any foam.
Lift out the chicken and let it cool for 10 minutes. Remove and discard the skin and bones and shred the meat into bite-size pieces.
Remove the bay leaves and thyme stems. Skim excess fat from the surface if you like.
Bring the broth back to a boil, add the noodles and cook according to the package, usually 6–8 minutes.
Return the chicken to the pot, stir in the parsley and lemon juice, taste for salt and serve hot.