Heat a large wok and add the oil. Once hot, add the vegetables a handful at a time and keep them moving in pan. Cook for 4-6 minutes.
Meanwhile, for the tuna steaks, heat a large griddle pan. Rub a little oil over the tuna steaks and season with salt and pepper. Cook each side for no more than two minutes (depending on how rare you like your tuna).
For the dressing, place all the remaining ingredients, except the mint and coriander cress, into a small bowl and mix.
To serve, place the stir-fried vegetables in the centre of serving bowls and place the tuna on top. Spoon the sauce over the tuna and garnish with coriander cress.