Sift the flour in a bowl, add the milk, a pinch of salt and use a manual whisk to knead them. Add a beaten egg and the melted butter at last. Carefully stir, cover the bowl and let the mixture rest for 30 minutes.
Roughly mince the hazelnuts with a knife. Cut the leek into rounds and stir-fry them in a pin, adding salt and pepper.
Put a non-stick pan for crêpes on the fire, melt a pinch of butter and pour a small ladle of the batter, covering the bottom of the pan. Tilt the pan so as to spread the batter over the whole surface.
Let it cook for one minute and, once the crêpe is golden, flip it over and wait for the other side to be golden, too.
Once the crêpe is ready, transfer it to a place. Repeat the same process for the remaining crêpes, and create a pile.
Stuff each crêpe with cooked ham, robiola cheese, stewed leeks, and hazelnuts.
Place the crêpes on a sheet pan covered by parchment paper, spread Grana Padano cheese on top of them and grill them in the oven for a few minutes.
Serve immediately these crêpes! with cooked ham!