1. Rinse quinoa thoroughly in cold water and drain well. Place quinoa in a large pot with 2 cups water. Bring to a boil over high heat. Reduce heat to medium, cover and simmer until most of the liquid has absorbed and quinoa is tender, about 15 minutes. Remove from heat and let stand for about 5 minutes. Fluff with a fork, transfer to a large bowl and refrigerate until cool, stirring occasionally.
2. Place eggs in a pot, cover with cold water and bring to a boil over high heat. When boiling, remove from heat, cover and let stand for 10 minutes. Run eggs under cold water to cool; peel and quarter lengthwise.
3. Bring a large pot of salted water to a boil over high heat. Add green beans and blanch until tender, about 2 minutes. Drain, run beans under cold water and drain again. Cover and refrigerate.
4. Whisk together lemon juice and mustard in a small bowl. Slowly drizzle in oil, whisking constantly. Stir in capers. Season with pepper.
5. Spread out quinoa on a serving platter. Top with tuna, green beans, tomatoes, olives, onion and eggs, forming rows of each. Drizzle with dressing, season with salt and pepper and serve.