Add oil to a large non-stick skillet over medium heat. Add the onion and sausage and cook for 5 minutes until the onions are starting to soften.
Add the broccoli and cook, covered, for another 5 minutes. Add the garlic and orzo. Cook for 1 minute.
Add the orzo, lemon zest, juice, and chicken stock. Heat to a heavy simmer. Turn down the heat to a simmer and cover. Cook for 10 minutes or until orzo is tender, stirring occasionally. If there is extra liquid, remove the lid for the last couple of minutes. Add the mozzarella cheese and stir it in as it melts.
Stir in the Parmesan cheese. Season with salt and pepper to taste.