Peel the potatoes and slice them thinly using a mandolin.
Do the same with the onion, and put it all in a big bowl.
Mix the eggs with the flour, then gradually pour the milk. Add salt and pepper.
Pour the batter in the potatoes and mix well. Grease a springform pan, and put half the potatoes in it.
Arrange the cheese and the prosciutto on it, then cover with the remaining potatoes. Smooth the top.
Bake 1 hour at 350°F (180°C).
There you are, your Raclette and potatoes cake is ready !