In a large bowl, mix the flour with the salt and baking soda.
Add the lard (or olive oil) and mix until the fat is well incorporated into the flour, producing something that looks and feels a bit like pebbly sand.
Now add the water, as much as you need to form a soft but not at all sticky ball. Knead well until you have a smooth and pliable dough.
Let the dough rest for about 30 minutes, wrapped in plastic wrap or a thick towel.
Divide the dough into balls weighing anywhere between 75g (2-1/2 oz) and 140g (5 oz), pending on how large you want your piadine to be (see Notes).
Take each ball and flatten out with a rolling pin or tortilla press into round disks. If you want perfect circles, you can trim the rounds with a pastry ring.
Lay your piadine out on a board, cover them with a towel and let rest them for another 15 minutes or so.
Now heat a griddle or non-stick skillet over a high flame. When when it's good and hot, lower the flame to medium and cook each piadina about 2 minutes on each side. (Meanwhile, keep the rest under the towel so they don't dry out.) They should be nicely spottled on both sides.
Serve slightly warm, with the fillings of your choice.