Combine prawns, oil, garlic, rind, juice and cumin in a bowl, mix well to coat, cover and marinate in the refrigerator for 20 minutes.
Meanwhile, for pea tabbouleh, cook quinoa in a saucepan of boiling salted water until tender (10-12 minutes), drain through a fine sieve and rinse under cold running water to cool.
Drain well, spread on a tray to dry out a little (15 minutes), then combine in a bowl with peas, cucumber, mint and spring onion.
Whisk olive oil, lemon juice, vinegar and garlic in a bowl to combine, season to taste and toss through salad just before serving.
Preheat a barbecue to medium-high heat.
Drain prawns from marinade, thread onto skewers and grill, turning occasionally, until cooked through (2-3 minutes).
Dust with sumac and serve hot with pea tabbouleh, yoghurt and lemon wedges.