Steam or microwave the spinach until just wilted. Refresh in cold water. Drain and squeeze out any liquid. Combine the ricotta, nutmeg and Parmesan. Season with salt and pepper.
Divide the pasta dough in half. Roll out each half into a sheet (see
Spread the ricotta mixture on to the pasta, leaving a 1.5 cm (¾ inch) border on the right and left. Scatter evenly with the spinach, then lay the Parma ham slices lengthways along the long edge nearest to you.
Brush the edges with water, then roll up the pasta, lifting the tea towel to help you. Pinch the edges to seal, then wrap in the tea towel. Tie both ends firmly with string and in 2–3 places along the length.
Cook the roll for 30 minutes in a fish kettle or deep baking dish of salted simmering water, using the kettle rack or a heatproof plate to keep the roll submerged.
Unwrap the roll, cut into 12 slices and arrange on individual plates. Drizzle with the melted butter and serve with a scattering of grated Parmesan.