Stir the balsamic vinegar, garlic, honey, mustard powder and salt and pepper together in a mixing bowl. Gradually whisk in the olive oil and let sit.
Fill a medium saucepan with water and add the wheatberries and 1/2 tsp. salt. Bring to a boil and simmer for about 10 minutes. Then add the barley and continue to simmer uncovered for another 20 minutes, until the wheat berries and the barley are almost al dente.
Drain and rinse under cold water to stop them from cooking and transfer to a bowl and let cool completely. Then stir in the dates, figs and grapes.
Add the red onions, scallions, garlic, parsley, olives and pomegranates. Put the greens out on a flat plate, cover with the salad, sprinkle on the feta or goat cheese, if using, and drizzle some of the dressing over. Toss and serve immediately.
You may find you need to cook the wheatberries on their own, for longer, before adding the barley. We recommend simmering the wheatberries on their own for at least 20 minutes before adding the barley.