Heat oil in a large pot and cook onion and carrots for 5 minutes. Add garlic and stir for 30 seconds.
Add chicken, broth, salt and pepper. Simmer gently for 18 to 22 minutes until the thickest part of the chicken reaches 165°F.
Remove the chicken to a plate. Stir orzo into the broth and cook for 8 to 10 minutes, or its package time, until tender.
Shred the chicken while the pasta cooks and return it to the soup.
Remove from the heat and stir in lemon juice and chopped dill. Serve promptly while the orzo still has a little bite.