Add half the tuna, egg yolk, gochujang, sugar, lime juice, 100ml water, ½ tsp salt and a few twists of black pepper to a small blender. Blitz until smooth, then pour in the butter and blitz again until emulsified.
Bring a medium pot or lightly salted water to a boil. Add the udon and cook for 2 minutes (or according to the packet instructions) then drain and place back in the pan and reduce the heat to low. Pour in the sauce and mix well until coated and glossy. Mix in the cucumber, coriander and remaining tuna.
To serve, divide between two bowls and sprinkle over the crispy onions and chilli flakes, if using.