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1
To make the sauce: Melt butter in a 1 1/2 to 2-quart heavy saucepan over moderately low heat.
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2
Whisk in flour and cook the roux, whisking, 2 minutes.
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3
Add the milk in a stream, whisking, and bring to a boil over high heat, whisking constantly (sauce will thicken).
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4
Reduce heat and simmer, whisking occasionally, 2 minutes, then whisk in salt, pepper, and nutmeg.
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5
Remove from heat and whisk in cheese, then cover pan.
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6
To make the cannelloni: Heat oil in a 5 to 6-quart heavy pot over moderately high heat until hot but not smoking, then saute onion and garlic, stirring occasionally, until lightly browned, about 5 minutes.
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7
Add spinach and saute, stirring, until just wilted, about 3 minutes.
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8
Remove from heat and cool completely.
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9
Stir together ricotta, egg, parsley, prosciutto, if using, salt, pepper, and 1/3 cup cheese in a bowl, then stir in spinach mixture.
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10
Boil pasta 2 pieces at a time in a 6 to 8-quart pot of boiling salted water, stirring to separate, until just tender, about 2 minutes for fresh pasta or about 6 minutes for oven-ready noodles.
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11
Gently transfer with a slotted spoon to a large bowl of cold water to stop cooking, then remove from bowl, shaking off water, and lay flat in between layers of plastic wrap.
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12
(Trim oven-ready noodles, if using, as closely as possible to 6 1/4 by 5 1/2-inch rectangles.)
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13
Preheat oven to 425 degrees F.
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14
Spread 2/3 cup sauce in buttered baking dish.
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15
Spread about 1/3 cup ricotta filling in a line along 1 short side of 1 pasta rectangle, then roll up to enclose filling.
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16
Transfer, seam side down, to baking dish.
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17
Make 7 more cannelloni in same manner, arranging snugly in 1 layer.
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18
Spread 1/2 cup more sauce over cannelloni and sprinkle with remaining cheese.
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19
Bake, covered with foil, in middle of oven until sauce is bubbling, about 20 minutes.
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20
Turn on broiler.
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21
Remove foil and broil cannelloni about 5 inches from heat until lightly browned, 2 to 4 minutes.
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22
Let stand 5 minutes before serving.
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23
Reheat remaining sauce and serve on the side.
