Make the tomato sauce, using only half the recipe.
If using fresh spinach, wash well, cook with salt until tender, drain, then squeeze out as much moisture as possible and chop coarse. If using thawed frozen leaf spinach, cook covered with salt about 5 minutes, drain, squeeze very dry, and chop coarse.
Saute the onion in 3 tbsp (42 g) butter until pale gold. Add the prosciutto and cook about 30 seconds, then add spinach and cook 2 minutes or more, until it has absorbed the butter.
Move contents to a bowl. Add ricotta, 1 cup Parmesan, about 1/8 tsp nutmeg, and egg yolk. Mix well and correct salt.
Roll the pasta very thin.
If using a machine, join strips into one large sheet by moistening and overlapping edges about 1/8 inch (3 mm), pressing firmly, then smoothing with a rolling pin. Trim irregular edges. If rolling by hand, use your single large sheet.
Spread filling over the pasta, leaving a 3-inch (8 cm) border near you and a 1/4-inch (6 mm) border on the other sides. Fold the 3-inch border over the filling, then keep rolling loosely until you have a large roll.
Wrap tightly in cheesecloth and tie both ends. Boil in 3 to 4 quarts (2.8 to 3.8 L) water, adding 1 tbsp salt. Cook at a steady, moderate boil for 20 minutes. Lift out carefully so it does not split. Remove cheesecloth while hot and let cool.
Preheat the oven to 400 F (205 C).
Make the bechamel sauce to medium thickness, then mix it with the tomato sauce.
When the roll is cool and firm, slice into 3/4-inch (2 cm) slices.
Lightly sauce the bottom of a bake-and-serve dish. Arrange slices in one layer, slightly overlapping like shingles. Pour the remaining sauce over, sprinkle with the 1/3 cup Parmesan, and dot with butter.
Bake on the top rack 10 to 15 minutes, until lightly golden. Rest 10 minutes and serve from the baking dish.