Pour broth into large saucepan set over high heat; bring to boil. Add quinoa and cook until barely tender, about 7 minutes.
Reduce heat to medium-low. Whisk eggs with lemon juice until frothy. Ladle about 1 cup (250 mL) hot broth into egg mixture. Whisk mixture back into simmering broth in saucepan.
Cook, stirring, until soup is slightly thickened, 1 to 2 minutes. Season with salt and pepper, if desired. Garnish each serving with parsley and Parmesan.
Tip: Substitute quinoa for orzo or rice.