Set the oven to 190 C | 375 F. You will need a baking dish. I used a 32cm round oven proof frypan but any baking dish will work. it should be big enough that you can put all the chicken in one layer.
Put the cornflour into a large bowl and add the chicken. Toss the chicken into the cornflour and roll it around to coat. Add the egg white and toss until the chicken and cornflour now looks like a thin batter. This will both thicken the sauce and make a nice coating on the chicken when it's baked. Set aside
To make the sauce grate the ginger finely and crush the garlic. Add both to the baking dish along with the soy sauce, tomato sauce, sugar, pineapple juice, and rice vinegar. Stir together well. You could do this in a bowl and then pour it into the baking dish but why make more dishes?
Add the roughly diced onion and capsicum (pepper), stir again. Put the chicken pieces on top of the sauce. I make sure you rub that chicken onto the sides of the bowl and get all the cornflour mix stuck to the sides onto that chicken. The chicken should be sitting above the sauce. Half in half out
Put the dish into the oven. Bake for 35 minutes, before piecing the centre of the biggest piece with a knife or skewer to make sure it is cooked through. This of course will depend on the size pieces you have, your oven, and how spaced out the pieces are. The boneless chicken will not take as long again. Please check chicken, if still pink give it another 10 minutes.
Take the tray out of the oven and turn each piece of chicken over in the sauce to coat and then bake again for 5 to 10 minutes.
Serve with chopped cucumber, pineapple and coriander and rice of course!