1. Warm oil in a medium saucepan over medium heat. Add onion and cook, stirring, until softened, 3 to 5 minutes. Add quinoa; cook, stirring, for 1 minute.
2. Add 2 1/4 cups water and 1/2 tsp. salt to pan. Turn heat to high; bring to a boil. Cover, reduce heat to low and cook until quinoa is tender, about 15 minutes, stirring in shrimp during final 5 minutes. Lift cover after 15 minutes and check that shrimp is cooked through and most of water has been absorbed. If not, cook 1 minute or 2 longer. Remove from heat, cover and let stand for 10 minutes.
3. Transfer mixture to a serving bowl. Stir in parsley, lemon zest and juice. Season with salt and pepper and serve.