Finely chop the onion, optional celery, courgette, and garlic.

Bring 1.5 L water and the 3 chicken bouillon cubes to a boil in a large pot.

Add the boneless chicken thighs and simmer until cooked through, about 15 min.

Remove the chicken from the broth and shred it into bite-size pieces.

Add the chopped onion, celery, courgette, garlic, and za'atar to the simmering broth.

Cook the vegetables until slightly tender, about 8 min.

Add the orzo pasta and cook until tender, about 8 to 10 min.

Whisk the 2 eggs and lemon juice together in a mixing bowl until smooth.

Whisk one ladle of hot broth into the egg mixture to temper it.

Remove the pot from the heat and slowly stir in the tempered egg-lemon mixture.

Return the shredded chicken to the soup and stir until warmed through.