Put the grated potatoes and courgettes in a clean tea towel and squeeze out as much water as you can. Tip into a bowl with the spring onions, egg yolk, flour and plenty of seasoning. Mix well.
Heat oven to 180C/160C fan/gas 4. Put a little oil and butter in a frying pan. Divide the mixture into 4 flat röstis and add to the pan (fry as many as you can fit in your pan at once). Cook over a medium heat for 6-8 mins until the underside is crisp and brown, then flip over and cook the other side. When they’re cooked, transfer to a baking parchment-lined tray and put in the oven.
While the röstis are finishing off in the oven, fry the bacon in the pan, then transfer to the oven, too. Finally, fry the eggs to your liking, then serve with the courgette röstis, crisp bacon and a good blob of tomato ketchup or chutney.