Make the dressing: Combine all dressing ingredients (except oil) in a blender and blend until smooth. While blender is running, remove lid and pour in oil in a steady stream. Continue blending for several seconds, then remove and set aside. Boil the prawns in slightly salted water for 3-5 minutes. Strain well; squeeze over juice of half lemon and season with salt and freshly ground black pepper. Heat some olive oil in a frying pan and fry the shrimps for another 3-5 minutes. In a large bowl, combine mango,avocado and red onions, add dressing to taste; toss lightly and transfer into serving plates, over green leaves. Top with prawns and serve.