Meanwhile, in a pot over medium heat, soften the onion and garlic in the oil. Add the orzo and 2 cups (500 ml) of the broth. Bring to a boil and let simmer for 6 minutes, stirring frequently with a wooden spoon. Add the broccoli and the remaining broth. Continue cooking for 6 minutes or until the orzo is al dente and the broccoli is tender. Off the heat, add the Parmesan and lemon zest. Season with salt and pepper. Let rest for 2 minutes.