Lomo saltado Beef stir fry

Name: Lomo saltado (Beef stir-fry)
Ingredients: 300g fillet steak, cut into large cubes; 1 tbsp vegetable oil, for frying; ½ large red onion, cut into thick wedges; 1 tomato, halved, deseeded and cut lengthways into eighths; 1 amarillo chilli or medium red chilli, deseeded and sliced lengthways; 1 tbsp Pisco (grape brandy) or vodka; 2 tbsp ají amarillo chilli paste or Basic chilli paste (see tips); small handful coriander leaves, finely chopped; chunky chips, to serve (optional); 2 garlic cloves, chopped; 75ml soy sauce; 1 tbsp red wine vinegar; 1 tsp Worcestershire sauce; ½ tsp dried oregano; ½ tsp ground cumin