Heat the olive oil in a saucepan, add the onion and garlic and fry for 2-3 minutes, or until softened but not coloured. Add the tuna and fry for 1-2 minutes. Stir in the chopped tomato, reduce the heat and simmer until the tomato has softened.
Add the soy sauce and a splash of water, stir in the parsley and season with salt and freshly ground black pepper. Add the cooked pasta to the pan and stir well to coat.
Spoon the pasta into a bowl and serve.