Prepare your prawns: use a pair of kitchen shears to cut through the back of the shell. Cutting through the shell lets you de-vein the prawn easily. In a large bowl, mix together the lime juice, fish sauce and sugar until the sugar is dissolved. Add the zest, chill and cilantro. Taste and season with salt and freshly ground pepper.
Heat up a cast iron over medium-high heat. Add a bit of olive oil to the pan. When hot, add the prawns and cook without moving for 2 minutes. Flip and continue to cook for 1-2 minutes, until pink and firm. Toss the shrimp in the dressing. Enjoy hot.
