1) Preheat the oven to 190C/Gas 5.
2) In a large frying pan, heat the oil over a medium-high heat. Add the potato cubes and season with salt and pepper, to taste. Saute stirring frequently, for 5 minutes.
3) Turn the heat up to high, add the water, stir, and cover the pan with a lid. Steam the potatoes until the water evaporates, about 3 or 4 more minutes. (Every 1 to 2 minutes, open the lid long enough to stir the potatoes so they don't stick.)
4) Lower the heat to medium, and saute another 1 to 2 minutes until all traces of water are gone. Toss in the butter and stir to coat the potatoes.
5) Spread the potatoes out on a baking tray. Roast in the oven until the potatoes are crisp and browned to your liking, about 15 to 25 minutes depending on how dark you want them. Transfer the potatoes to a serving bowl and serve.