Cook the quinoa according to the package instructions. Drain off any excess water and refrigerate until cool.
Dice the tomatoes and place them in a mesh strainer that is resting over a bowl. Sprinkle them with 1/2 teaspoon salt and mix to combine. The salt prevents sogginess by releasing water from the tomatoes into the bowl.
Finely chop the green onion and set aside.
Trim the thick stems off the parsley, then place about 1/2 of the bunch in your food processor. Process until very finely chopped. Measure out 2 cups and place it in a serving bowl.
Add the cooled quinoa, green onion, and mint (if using) to the bowl. Grab the mesh strainer with the tomatoes and give it a little shake over the bowl to release excess water, then add them to the serving bowl.
In a small bowl, whisk together the olive oil, lemon juice, lemon zest, 1/2 teaspoon salt and black pepper. Pour it in the serving bowl and toss well to combine.
Taste and add an extra squeeze of lemon juice or more salt if needed. Let the flavors meld for 10-20 minutes if you can force yourself to wait, serve immediately. Store any leftovers in an airtight container in the fridge.