Place the first 11 ingredients, using only 2 Tbsp. of the oil, in a food processor and pulse until well combined and vegetables are finely chopped. Transfer to a bowl; season with salt and pepper. Chill in the fridge at least 3 hours. Toss the prawns with the remaining oil, salt and pepper. Grill over high heat until just cooked through, about 1 to 2 minutes per side; chill well. Ladle the soup in chilled bowls or large martini glasses. Divide and hook the prawns on the rim. Garnish with cilantro sprigs and serve.