This is from my own recipe book, with some comments added, I discovered recently, by my mother. I'll set out the (terse) recipe, followed by her commentary.
1 onion, chopped.
Herbs (eg rosemary).
Salt and peppper.
1. Heat oil in large frying pan.
2. Add onion; fry until soft; add herbs.
3. Add ham; fry until browned (don't let onion burn).
4. Add potatoes; fry until crisp.
5. Add salt and pepper. Serve.
6. Raw egg yolk in middle of plate. Allow to cook. Eat.
"Alan Miller", wrote my mother, "worked for the BBC and was from Scotland. He stayed with us for several months and made this. Ali's version is different; she uses butter and a tiny bit of olive oil to stop it browning and she doesn't add any herbs. And the potatoes must be pre-cooked and cut into tiny cubes. The egg yolk should be settled into the mound of potatoes like an egg in a nest. You could make a less delicate version with chorizo."
My own commentary: this is, I think, originally a Swiss dish, introduced to us by Alan Miller, who gave me my first ride on the back of a motorcycle: I can still remember the raw terror. I agree that the potatoes should be pre-cooked and cut into tiny cubes. The idea of a raw egg yolk puts many people off. Don't let it. Because provided you are organised, the egg yolk cooks in the middle of the potatoes. Having nested the egg yolk, I then pile potatoes around it (carefully so it does not break) and then start eating from around the edges before attacking the egg yolk. I have been known to add a dash of tabasco and even a squeeze of lemon juice to this.