Boil your potatoes in salted water for 10 minutes until almost cooked. Then drain and cool. I popped mine into the fridge for 10 minutes. Fry your onions for 10 minutes until softened.
Once your potatoes are cool, coarsely grate them, and add them to your onions in a bowl. Then add the egg yolk and season well.
Heat your olive oil and butter in a frying pan and divide your potatoes in half. Press half the potatoes into a large ball and then place into the frying pan and press down using a spatula to make a flat, round rosti. Tidy up the edges by pushing them together. Repeat with the rest of the potato. Fry for five minutes on each side. Be careful when flipping over as they can be delicate.
Meanwhile, fry your bacon until cooked in a pan. Remove and set aside. Add your leeks to the pan and fry for 10 minutes until cooked through. Then return the bacon to the pan and mix.
Meanwhile, make a cheese sauce by melting the butter and then stirring in the flour to make a roux. Add the milk little by little, stirring all the time until you have a nice thick sauce. Remove from the heat and crumble in your blue cheese. Pour your cheese sauce into your leeks and bacon and stir to combine. Serve dolloped over your crispy rosti.