Heat olive oil over medium-high heat in a dutch oven or soup pot. When oil shimmers, add chicken and sprinkle with 1 tsp salt. Cook, allowing chicken to brown, until cooked through, about 5-7 minutes. Transfer chicken to a separate dish and set aside.

In the same pot, add onion, celery, carrot, and thyme. Cook over medium heat until vegetables begin to soften, 5-10 minutes. Add 5 cups of chicken stock, reserving one cup. Bring the soup to a boil, add the orzo, and cook until orzo is al dente, about 10 minutes. Add reserved chicken and turn the heat down to low.

In a separate small pan, heat the reserved cup of stock until steaming (not boiling). In a small bowl, beat the egg, then mix in lemon juice and zest. Slowly pour in the hot stock, stirring constantly, then whisk the egg-stock mixture into the soup. Return the soup to low-med heat until soup is heated through and thickened slightly (do no allow the soup to boil or you’ll risk scrambling the eggs). Season to taste with salt and pepper and serve hot in bowls, garnishing with thyme sprigs if desired.