Step 1: Chop your chicken breasts into approx. 2cm pieces, and slice your onion.
Step 2: Cook your chicken in 1 tbsp of oil in a saucepan or frying pan. (I find saucepan easier.)
Step 3: Once cooked add your onion, and leave to cook for approx 5 mins.
Step 4: Drain the juices from your pineapple into a measuring jug. Put the pineapple on 1 side.
Step 5: Fill te jug up to 250ml with water and stir.
Step 6: Add 1 tbsp cornflour, 1 tbsp vinegar, 1 tbsp soy sauce, 1tbsp sugar, 1 tbsp lemon juice and 1-2 tbsp tomato puree to the syrup, stirring after each addition.
Step 7: Pour your sauce into the saucepan/frying pan you are cooking your chicken in.
Step 8: Cook for approx 15 mins. About 10-12 minutes in, add as much of the tinned pineapple as you wish.