Preheat oven to 180C.
Cook the quinoa: In a saucepan, bring quinoa and water to a boil. Reduce the heat to a simmer, cover, and cook until most or all of the water is absorbed, about 12-15 minutes. If any liquid remains, strain and rinse it under cold water to stop the cooking process.
Toss onion slivers with 1 tablespoon olive oil, salt and pepper. Roast for about 30 minutes. Resist stirring until they begin to brown, then stir occasionally; not too much or they will not brown as nicely. Remove when they are soft and nicely coloured. Let cool.
Pit and dice mangoes.
Make the dressing: Whisk the olive oil into the lemon juice. Whisk in the ginger and add salt and pepper to taste.
Assemble the salad: Mix the quinoa, mango (and juices), black beans, and coriander together. Spread the mesculan or greens on a large plate and layer the quinoa mixture over it. Top with the roasted onions and the avocado slices. Drizzle the dressing over the salad and serve.