Take a large saucepan and over a medium heat, melt the butter, sauté the onions, carrots, celery and leeks until softened, 8-10 minutes.
Add the garlic and bay leaves, cook for a minute or so. Pour in the chicken stock and stir. Bring to a boil.
Stir in the orzo and simmer for 8-9 minutes or until the orzo is just al dente. I tend to keep the lid off and stir it fairly often because the orzo can stick to the bottom of the pan.
Add in the cooked chicken and heat through. Season really well - it needs plenty of salt and a good grind of pepper.
Squeeze in the lemon juice (you could add more if you want it more lemony). Take off the heat, stir through the herbs, taste and season. Serve immediately.