In 4-quart saucepan or stock pot, sauté onions and garlic in olive oil until onions are translucent. Stir in celery and carrots; sauté 5 minutes.
Add chicken stock; bring to a boil over high heat. Stir in sliced sausage, orzo and rosemary. Reduce heat and simmer until orzo is tender, about 10 minutes, stirring occasionally.
Stir in lemon juice; season with salt and pepper to taste. If desired, garnish with parsley before serving.