Add 1/2 cup of quinoa to 1 cup of water. Bring to a boil, then lower temperature.
Cook until water has been fully absorbed, about 10-15 minutes. Remove from heat and allow to cool.
Prep all other ingredients, except for the last 5 ingredients, and add to a large bowl.
When the quinoa has cooled, add it to the bowl and mix.
Juice the lemon and add it to the bowl with a drizzle of olive oil. Season with salt and pepper (plus, a sprinkle of red pepper flakes, if desired) and stir to combine.
Add crumbled feta cheese on top before serving.
If not serving immediately, chill in the fridge in a covered container.