In a large mixing bowl, whisk together the flour, baking soda, and salt. Make a well in the center.

Pour the melted lard (or olive oil) and warm water into the well. Gradually incorporate the flour from the sides, mixing with your hands until a shaggy dough forms.

Turn the dough out onto a lightly floured surface and knead for 8-10 minutes until it's smooth, elastic, and no longer sticky. If the dough is too sticky, add a tiny bit more flour; if too dry, a tiny bit more water.

Form the dough into a ball, place it back in the bowl, cover with a damp kitchen towel, and let it rest at room temperature for at least 30 minutes (up to 2 hours). This allows the gluten to relax, making the dough easier to roll.

Divide the rested dough into 4 equal pieces. Roll each piece into a ball.

Lightly flour your work surface and a rolling pin. Take one dough ball and roll it out into a very thin, round disc, about 8-10 inches (20-25 cm) in diameter and about 1/8 inch (3 mm) thick. It doesn't need to be perfectly round. Repeat with the remaining dough balls, keeping the unrolled dough covered.

Heat a cast-iron skillet or non-stick griddle over medium-high heat until hot. Do not add oil.

Place one piadina onto the hot skillet. Cook for 1-2 minutes per side, or until golden brown spots appear and the piadina puffs up slightly. Prick any large bubbles with a fork to prevent excessive puffing.

Remove the cooked piadina from the skillet and stack it on a plate, covering it with a clean kitchen towel to keep it warm and pliable while you cook the remaining piadine.

Once all piadine are cooked, take one warm piadina. Spread about 1 tablespoon of stracchino cheese evenly over one half of the piadina.

Layer a few slices of prosciutto crudo over the cheese, followed by a handful of fresh arugula.

Fold the piadina in half, gently pressing down. Serve immediately. Repeat with the remaining piadine and fillings.