Optional step: To reduce the sharp flavor of raw onion, place the chopped red onion into a sieve and rinse with water for about a minute. Alternatively, you can leave the chopped onion in a bowl of water to soak for around 5 minutes, then drain and pat dry.
Cook your choice of starch (quinoa, barley, couscous, etc) in a pot on the stove according to package instructions.
In the meantime, chop the cucumber, parsley, mint, and toasted pistachios and place into a large bowl. Once the barley has finished cooking, add it to the bowl and crumble over the feta. Use a spoon to evenly mix all of the salad components.
Combine all of the dressing ingredients in a small jar or deli container. Screw on the lid and shake vigorously to combine, then pour the dressing over the salad and toss to combine. Taste to adjust for seasonings, adding more salt, chopped herbs, and lemon juice to your preference.
Cover and set the salad in the fridge for at least 30 minutes for the flavors to meld, then serve with your choice or protein or as a side dish (if needed, the salad may be served immediately). Store any leftovers in an airtight container for up to 5 days in the refrigerator.