Spiralize the carrot and cucumber and put in a serving bowl in little mounds. Thinly slice the mango and add to the bowl. Trim the ends from the snow peas and boil some water to cook them.
Warm the 1tsp oil in a small skillet over a medium-high heat. Add the tuna and cook for a couple minutes on each side to sear. Cook a little more if you prefer, but tuna works underdone as well. Remove from pan and rest a minute before slicing.
Meanwhile, put the snow peas in a small pan with boiling water and boil for no more than a minute. Drain and run under cold water to stop them cooking further. Add them to the bowl along with the sliced tuna.
Whisk/shake together in a small jar the ingredients for the dressing and pour over the bowl.