Preheat oven to 425 degrees. Cook grains; mix them in a large serving bowl.
Cover onions with hot water, and soak for 2 minutes. Peel and halve onions. Place on a rimmed baking sheet, and toss with 1 teaspoon oil, 1/2 teaspoon salt, and 1/8 teaspoon pepper. Roast until caramelized, about 20 minutes. Pour cider over onions, and roast until liquid reduces by half, about 5 minutes. Let cool slightly.
Gently toss grains with onions, vinegar, feta, parsley, lemon zest, and the remaining oil, salt, and pepper. Stir in apple, and serve immediately.
This salad's diverse grains complement one another quite nicely, although you can make it with any combination you prefer. To save time, boil both types of quinoa in one pot, the semi-pearl farro and pearl barley in another, and the purple barley in a third.