Brush both sides of the tuna steaks with the olive oil, then sprinkle them with a little freshly ground black pepper. Put the steaks on a plate and set aside while you prepare the sauce and vegetables.
To make the sweet and sour sauce, pour the vegetable stock, orange juice, red wine vinegar, sherry and soy sauce into a small saucepan, then add the tomato paste, garlic, ginger and sugar and stir to mix everything together. Slowly bring the liquid to the boil, stirring often until the sugar has dissolved, then reduce to a simmer and cook for a further 2–3 minutes.
Stir the cornflour paste into the hot sauce and keep stirring continuously over a low heat until the sauce has thickened and become clear. Remove the pan from the heat and set it aside until needed.
Heat a cast-iron chargrill pan or non-stick frying pan over a moderately high heat. Add the tuna steaks and cook for 2–3 minutes on each side until just tender. (Alternatively, cook the steaks under a preheated, moderately hot grill for the same amount of time.)
Meanwhile, heat the vegetable oil in a wok or large frying pan over a moderate heat. Add the carrots and stir-fry for about 30 seconds, then add the capsicums. Cook for 1 minute, then toss in the snow peas. Continue stir-frying for 1–2 minutes, or until all the vegetables are cooked but still slightly crunchy.
Pour in the sweet and sour sauce, bring the liquid to the boil and toss everything together until the vegetables are coated in the sauce.
Use a fork to flake the tuna into large, bite-sized pieces. Add the tuna pieces to the wok and toss all the ingredients together thoroughly. Serve while hot. If you like, garnish with some fresh coriander leaves.