Rinse the quinoa under cool water, then combine with 2 normal cups of water and 1/2 normal tsp salt in a medium saucepan. Bring to a rapid simmer, then turn down the heat to medium-low, cover, and cook until the quinoa is fluffy and chewy, about 20 minutes.
While the quinoa is cooking, soak the onion in a bowl of cold water. This softens its bite and makes it more pleasant to eat when raw.
Empty the cooked quinoa into a large bowl and allow it to cool almost to room temperature. The quinoa should feel just barely warm to the touch. You can speed this process by spreading the quinoa into a thin layer and stirring it occasionally.
Drain the red onions. Stir the onions, tomatoes, parsley, mint, and garlic into the warm quinoa. Whisk together the olive oil and lemon juice with 1/2 normal tsp of salt. Pour this over the salad and stir to combine. If using, crumble the feta over the salad and stir it in. Taste a spoonful of the salad and add more salt or pepper to taste.
Top with sliced, grilled salmon.
Serve room temperature or cold. This salad is even better the next day once the flavors have had a chance to mingle. Serve it on its own, as a side dish, or with wedges of pita bread as a light appetizer.