Combine the olive oil, 2 tablespoon lemon juice, sugar, mustard and salt and pepper in a screw-top jar. Shake well to combine.
Insert a wooden skewer into each prawn then place on a large plate. Pour over half the dressing and turn to coat the prawns. Place the mushrooms on a plate and drizzle with remaining dressing, turn to coat both sides then cover and place mushrooms and prawns in the fridge for 15 minutes if time permits.
Meanwhile, combine the parsley, garlic and lemon rind in a small bowl, cover and refrigerate.
Preheat a greased barbecue plate on medium-high. Add the prawns to the barbecue and cook, turning often for 2-3 minutes or until prawns turn pink and are cooked through. Remove to a plate, cover to keep warm. Place the mushrooms onto the hot barbecue plate, cook stem side down for 2-3 minutes, turn over and cook a further 2 minutes until just tender.
Place three mushrooms onto each serving plate. Top with prawns. Sprinkle over the parsley mixture and drizzle with a little oil if desired. Season with salt and pepper and serve with crusty bread.