Cook 300g of waxy potatoes in salted water. In a decent non-stick frying pan cook some chopped onion in olive oil. Drain the potatoes, remove the skins, and then slice them up. Add the potatoes to the hot pan.
Whisk up four eggs, and season them with salt and pepper. Pour the egg into the pan, shaking the pan around so that the onion and potatoes begin to set with the egg. When the egg is almost set (this should only take just under a minute) place the pan under a hot grill. You want the top of the omelette to take on some colour (i.e. to turn golden brown), but at the same time, you don't want it to burn.
Turn the Spanish Omelette out onto a plate. The Spanish often serve it cut in to wedges.