In a large microwaveable dish, mix the ketchup, vinegar, sugar and garlic thoroughly with the chicken, onion and peppers. Microwave, uncovered, on high for 8-10 mins until the chicken is starting to cook and the sauce is sizzling.
Stir in the pineapple pieces and sugar snap peas and return to the microwave for another 3-5 mins until the chicken is completely cooked. Leave to stand for a few minutes, then stir in the cashews, if using, and serve.
