Mix the sauce ingredients together thoroughly and set aside.
Cut the block of tofu into small cubes. Put in a mixing bowl and sprinkle over the cornflour and plenty of salt and pepper. Toss together with your hands so that the tofu is coated in the cornflour.
Heat the oil in a large non-stick frying pan over a fairly high heat. Add the tofu and cook for about 6 minutes, tossing now and again, or until golden and crisp. Remove from the pan with a spatula, leaving the oil in the pan, and put on kitchen paper to drain.
Keeping the pan over a medium-high heat, add the pineapple chunks. Let them cook for 3-4 minutes, tossing once or twice, so they start to take on some colour.
Add the sliced peppers and cook for another 5 minutes or so, stirring or tossing now and again, until softened. Tip the sauce into the pan and stir with the pineapple and pepper. Cook for a couple of minutes so that the sauce is bubbling and reduced but not cooked away to nothing.
Spoon rice into 3-4 bowls. Divide the peppers and pineapple between the bowls, then add the crispy tofu. Sprinkle with thinly sliced spring onion and serve.