Boil the potatoes in boiling water for 5 minutes, or until you can easily pierce them with a fork.
Meanwhile, place the cubes of pancetta in a non-stick pan and sear them over medium-high heat, stirring occasionally, until they form a light crust. Remove the pancetta cubes from the pan and let cool in a bowl.
When potatoes are ready, drain them and let cool for 5 minutes to avoid burning your hands when peeling them.
While the potatoes and pancetta are cooking, prepare the dough. Add the flour to a large bowl and form a well in the center. Add the lard, salt and baking soda. Knead everything together by hand, then gradually add the milk while continuing to knead by hand until you have a firm, elastic dough.
Let the dough rest covered at room temperature for 20 minutes.
Once the potatoes are cooked and slightly cooled, peel them and mash them in another bowl with a potato masher.
Add the pancetta, Parmesan and nutmeg, and mix until well combined. Season with salt and pepper to taste.
Lightly flour the work surface and roll out the dough, not too thick otherwise it won’t cook properly, nor too thin as it could break under the weight of the filling. 2 mm should work well.
Spread the potato filling over half of the dough, leaving an empty border to seal well, and cover with the other half of the dough.
Press the dough surface with a rolling pin to evenly distribute the filling, then cut rectangles measuring 6-7 cm with a ravioli cutter.
Heat a non-stick or cast iron pan over medium heat. When the pan is very hot, place the Tortelli in it in a single layer and cook on both sides until golden brown (They should look like mini calzones). Remove the Tortelli and as you cook the next batch, place them in a container, preferably terracotta, to keep them warm. Repeat with the remaining Tortelli.
Your Italian street food "Tortelli alla lastra" is ready. Enjoy!