Sweet and sour stir-fried fish with ginger

INGREDIENTS
1–2 tbsp cornflour
salt and freshly ground black pepper
675g (11⁄2lb) thick white fish fillets, such as
haddock, cut into strips
1–2 tbsp vegetable or sunflower oil
1 onion, roughly chopped
2 garlic cloves, grated or finely chopped
2.5cm (1in) piece of fresh root
ginger, finely sliced
large handful mangetout or sugarsnap peas, sliced into strips

For the sweet and sour sauce
1 tbsp white wine vinegar
1 tbsp tomato purée
1 tbsp sugar
1 tsp cornflour
2 tsp light soy sauce
2 tbsp pineapple juice

METHOD
First, make the sauce. Mix together the vinegar, tomato purée, sugar, cornflour, soy sauce, and pineapple juice in a jug, and set aside.
Put the cornflour on a plate, and season with salt and pepper. Toss the fish in the seasoned flour to coat.
In a wok, heat about half of the oil until hot, then add the fish. Stir-fry for about 5 minutes until golden. Remove with a slotted spoon, and set aside to keep warm. Carefully wipe out the wok with kitchen paper, and add a little more oil. When hot, add the onion and stir-fry until it begins to soften, then add the garlic and ginger, and stir-fry for a few minutes more.
Pour in the sweet and sour sauce, and let boil for a few minutes, stirring constantly. Reduce the heat to medium, add the mangetout or sugarsnap peas, and stir-fry for 1 minute. Return the fish to the wok, quickly toss together to combine, and serve hot.