Place the monkfish in a bowl with the lemongrass, lime leaves, lemon thyme and olive oil. Cover and chill for 6 hrs to marinate.
Meanwhile, make the lemon dressing. Chop the strips of lemon zest into small dice and tip into a pan of boiling water. Bring back to the boil, then drain. In a small saucepan, cook the blanched lemon zest with the sugar in 140ml water for about 5 mins. Drain the lemon zest and mix together with the remaining ingredients and 2 tbsp water. Taste and season, if you like.
About 20 mins before you’re ready to eat, remove the monkfish from the marinade and lightly season. Heat some olive oil in a large non-stick frying pan over a medium heat and fry the monkfish medallions for 4 mins on each side until golden brown. Remove from the pan and allow to rest in a warm place for 4 mins. To serve, gently warm the dressing, adding more water if needed. Spoon it over the monkfish and around the plate, then scatter with rocket leaves.