Knead the potatoes - previously boiled, mashed and sifted - with flour, cornflour, a pinch of salt and the egg. Make gnocchetti and put them aside.
Melt the butter in a casserole to cook the onion, add the crumbled cotechino, the cinnamon and rosemary and simmer with white wine; then, let the sauce cook over a gentle heat for at least one hour, with the lid on.
Then, boil the potato gnocchi in plenty of salted water and drain them directly in the sauce, that should remain quite wet.
Top with the parmesan.