Watercress chicken stir fry

Name: Watercress & chicken stir-fry
Ingredients: 1 tbsp sunflower oil; 2 skinless and boneless chicken breasts , cut into strips; 100g cashew; 1 red or yellow pepper , deseeded and chopped into large chunks; 1 red onion , chopped into large chunks; 2 x 75g bags watercress; 3 tbsp hoisin sauce; 2 tbsp soy sauce; 1 large knob of fresh root ginger , peeled and finely grated; 2 garlic cloves , crushed; 1 tbsp sesame oil; 2 tbsp rice vinegar or white wine vinegar