In a large stockpot, heat 2 tablespoons of oil over medium high heat.
Season chicken breasts with salt and pepper to taste and dredge in the all purpose flour.
Shake off any excess flour and then add chicken to the stock pot.
Cook chicken until golden brown, about 2-3 minutes; set chicken on a plate aside.
Add the remaining 1 tablespoon of oil to the stockpot. Add the onion, carrots and celery to the stockpot and cook over medium heat for 3-4 minutes.
Add the sliced leek and garlic and cook for about 2 minutes and season with salt and pepper.
Add the chicken stock and water and bring to a boil.
Cover and simmer over low heat, stirring occasionally for about 20 minutes.
Add the orzo (or quinoa), return the chicken to the stockpot, cover and simmer for about 10 minutes or until orzo (or quinoa) is tender.
Stir in the lemon juice, season with salt and pepper and sprinkle with tarragon (or dill or parsley).