In a small saucepan over high heat, add water and cider vinegar and bring to the boil. Remove pan from heat then stir in honey and orange zest. Set aside to cool.
Using a potato peeler, slice thin strips of cucumber (avoiding the seeds). Place in a bowl and pour over honey cider vinegar. Cover and place in fridge to marinate for as long as possible, ideally overnight.
Combine sesame seeds in a shallow dish. Roll tuna in seeds to ensure all sides are evenly coated.
Heat oil in a heavy-based frying pan over medium-high heat. Add tuna and sear on all sides. Remove tuna from pan, wrap tightly in cling film and set aside while you prepare the rest of the dish.
Drain cucumber (reserving liquid) then roll slices in tubes. Carefully slice tuna as thinly as possible.
To plate, lay melon down first then tuck pieces of tuna into melon. Top with bocconcini then add cucumber tubes. Drizzle a few drops of orange balsamic over each plate (or use the cucumber marinating liquid), garnish with dill and serve.