SQUEEZE the excess moisture from the spinach and set aside.
MELT the butter in a saucepan and fry the onion until golden. Add the ham and fry until beginning to brown. Take off the heat and stir in the spinach. Cool slightly, then beat in the ricotta and the egg. Season to taste with salt, pepper and nutmeg.
ROLL the pasta out to a rectangle about 12 x 16 inches. Spread the filing all over leaving a 1/2-inch border all round the edge of the rectangle.
ROLL up the pasta and filling from the narrow end (your final result is as wide as the wide dimension of the pasta) and wrap in cheesecloth or a small dish cloth to form a sausage shape, tying both ends and the middle securely with string.
IN a pan large enough to accomodate the rotolo, bring enough lightly salted water to cover the rotolo to a boil, reduce heat to a gentle simmer and poach - the simmering water covers the 'sausage' - for about 20 minutes or until firm. Carefully remove, drain and then unwrap. Let cool.
WHEN you are ready to finish the dish preheat the oven to 400 F.
CUT the pasta roll into 1-inch slices. Spoon a little béchamel sauce over the base of a shallow baking dish and then arrange the slices on top so that they are slightly overlapping.
SPOON the remaining sauce over the rolled slices, sprinkle with the cheese and bake for 15-20 minutes or until browned on top and bubbling. Let stand for a few minutes before serving.