Heat 1 tablespoon of the olive oil in a medium sauce pan over medium-low heat. Add the onions and cook, stirring frequently, until soft and translucent, about 5 minutes.
Add the quinoa to the onions and continue cooking, stirring constantly, for 3-4 minutes. Add the vegetable broth and stir in 1/2 teaspoon salt. Turn the heat up to high and bring to a boil. Cover the pan tightly with a lid, turn the heat down to low and simmer for 17-20 minutes, or until liquid is absorbed and quinoa is cooked. Transfer the cooked quinoa to a mixing bowl and chill in the refrigerator.
When the quinoa is cool, add the remaining tablespoon of olive oil, tomatoes, corn, scallions, cilantro, remaining 1/2 teaspoon salt, lime juice, feta and jalapeño, if using. Taste and adjust seasoning if necessary. Right before serving, scatter the avocado chunks over top.
When corn is not in season, I use Trader Joe's organic white frozen corn. I add it after the quinoa has been in the refrigerator for about an hour - not hot, but still warm enough to thaw the corn.
Source: Once Upon a Chef