Rinse the barley thoroughly under cold running water until the water runs clear; cook uncovered in a large amount of salted boiling water until tender, about 45 minutes. Drain and rinse under cold water until completely cooled.
While the barley is cooking, preheat the oven to 425°F; combine the cauliflower florets, sweet potatoes, red onion, olive oil and spices in a bowl, mix well and arrange on a 18" x 26" baking sheet. Bake for 20 minutes then stir the veggies around, add the chopped walnuts and half the cooked barley to the pan and return to the oven for another 10 minutes.
Arrange the rest of the cooked barley in a serving dish and then top with the roasted veggies as soon as they come out of the oven.
Garnish with the chopped cilantro, lime and lemon wedges and serve without delay.