Cook the orzo according to the package directions; drain well, running under cool water to cool it down, then dump into a bowl.
Thinly slice your spinach: tack a bunch of leaves, roll them up tightly and slice through the whole bunch – or just tear it in with your hands. The method you choose will likely depend on the salad's final destination. Add the onion, feta and lemon zest.
Squeeze the lemon juice over the salad, and drizzle with the rice vinegar, oil, salt and pepper. Toss to coat well. Taste it and adjust the seasonings if it needs it. Serve right away or refrigerate until you’re ready for it.