In a large pot, heat the oil over medium heat. Add the carrots, celery, onion and garlic, and cook for 5-6 minutes, stirring often, until the onions have softened and started to turn translucent.
Add the broth, orzo, bay leaf, salt, oregano, basil, and thyme. Bring to a simmer and cook for 10-12 minutes until the orzo is tender.
Optional for slightly thicker soup: stir together the melted butter and flour until smooth. Whisk into soup and simmer for 1-2 minutes.
Stir in the chicken and lemon juice and cook until heated through, 2-3 minutes.
Remove the bay leaf. Season to taste with additional salt and pepper, if needed. Serve servings of soup with chopped fresh parsley and Parmesan cheese (optional but tasty).
Notes
Lemon juice: the amount of lemon juice you add will depend on your taste preference. I suggest starting with 1/4 cup and adding more to taste. Bottled lemon juice may alter the flavor of the soup; I suggest using fresh lemon juice if you can.Optional Add-In: it works really well to stir in several cups of chopped fresh spinach at the end.Orzo: you can add a full cup of orzo pasta for a less brothy soup. I haven't tried subbing rice for the orzo, but I'm guessing it will work just fine.Freezable: I haven't tried freezing this soup. If you want to try, I'd suggest undercooking the orzo just a bit so it doesn't get mushy after freezing and thawing.