Heat a griddle pan until hot.
In a frying pan, dissolve sugar in vinegar and 60ml of water.
Add lemongrass, chilli, mint, and coriander to the syrup.
Simmer for two minutes until sticky and coat the tuna in this glaze.
Griddle the tuna until bar marks appear on all sides while keeping the center pink.
Brush remaining glaze over the tuna and let it rest.
Blanch asparagus, radishes, and turnips in boiling water and drain.
Refresh the blanched vegetables in iced water.
Combine watercress, herb leaves, radish leaves, and blanched vegetables in a bowl.
Slice tuna into bite-size pieces and serve with the salad garnished with cress and extra glaze.