Cut the blocks of halloumi into cubes and add to a large, non-stick frying pan with a drizzle of oil. Fry on a medium heat for around 5 minutes, turning regularly until golden brown all over. Set the halloumi aside. Add the peppers and onion to the pan with a little more oil and fry for 10 minutes on a medium heat, stirring regularly until soft. In the meantime, make the sauce. Pour the pineapple juice into a bowl and stir in the cornflour until smooth. Add the ketchup, soy sauce, chilli powder, sugar, white wine vinegar, and season with salt and pepper. Stir until smooth. When the veg has finished cooking, pour the sauce in and simmer for around 3 minutes until thick, bubbling, and coating the veg. Pour in 200ml boiling water and return the halloumi to the pan with the pineapple chunks. Mix thoroughly and simmer for 3-5 minutes on a medium heat until the sauce reaches your desired thickness. Serve immediately with rice.