For the tuna steak, heat a griddle pan until hot.
Heat a frying pan and add the sugar, vinegar and 60ml/2fl oz of water. Once the sugar has dissolved, add the lemongrass, chilli, mint and coriander.
Cook for a couple of minutes until sticky and then roll the tuna in the glaze.
Place the tuna onto a griddle pan and cook until bar marks appear and turn the tuna around until you have marks all around the tuna. Make sure it is pink in the middle.
Once removed from the pan, brush the remaining glaze over the tuna. Allow to rest for a few minutes while you make the salad.
For the asparagus salad, heat a saucepan of boiling water and blanch the asparagus, radishes and turnips. Drain.
Place the vegetables in iced water to refresh.
Place the watercress, mint and coriander leaves in a large bowl. Add the blanched vegetables, along with the radish leaves and mix it all together.
To serve, cut the tuna into bite-size pieces and serve with the salad alongside. Garnish with baby coriander cress. Drizzle over any glaze that's left in the pan after cooking the tuna.