Place everything in a pot and bring it to a boil over high heat, mixing constantly with a whisk.
Remove it from the heat and let it cool down completely.
Cut the medallions in thick wedges. Add salt, pepper, sesame oil and mix.
Cover the wedges with toasted sesame seeds but avoid covering up the edges of the wedges so you can check as the tuna cooks as to avoid overcooking it.
Preheat a pan over high heat and add the oil. Cook the tuna for 20-30 seconds per side or until you see the tuna starting to change colour.
Remove the tuna from the pan and let it cool down completely before cutting into it. Alternatively, you can place it in the fridge for a couple of hours before cutting it if you want it cold.
Cut the tuna in 5mm slices and spread them on a dish like a carpaccio.
Drizzle the sauce generously on top and then sprinkle the avocado, cucumber, radish sprouts and chives.