Boil the diced potatoes in salted water for 8-10 minutes until just tender. Drain well and let them steam dry for a minute.
Heat the oil in a large frying pan over a medium-high heat. Add the onion and cook for 3-4 minutes until softened.
Add the drained potatoes to the pan and cook without stirring for 3-4 minutes until the underside is golden and crispy.
Flip the potatoes and cook for another 3 minutes. Add the cubed corned beef and mustard, stir gently to combine.
Press the mixture down flat with a spatula and cook for 5 minutes until the bottom is crispy. Flip in sections and crisp the other side.
Season well with salt, pepper and a splash of Worcestershire sauce if using. Serve with a fried egg on top if you like.