Watercress & chicken stir-fry

Name: Watercress & chicken stir-fry
Cook time: 10 min
Total time: 10 min
Cuisine: Asian
Ingredients: 1 tbsp sunflower oil; 2 skinless and boneless chicken breasts, cut into strips; 100g cashew nuts; 1 red or yellow pepper, deseeded and chopped into large chunks; 1 red onion, chopped into large chunks; 2 x 75g bags watercress; cooked rice, to serve (optional); 3 tbsp hoisin sauce; 2 tbsp soy sauce; 1 large knob of fresh ginger, peeled and finely grated; 2 garlic cloves, crushed; 1 tbsp sesame oil; 2 tbsp rice vinegar or white wine vinegar