source: Hamish and http://www.dishbase.com/recipe/spinach-cheese-cannelloni/
Sauce: Melt butter in a 1 ½- to 2-quart heavy saucepan over moderately low heat. Whisk in flour and cook roux, whisking, 2 minutes. Add milk in a stream, whisking, and bring to a boil over high heat, whisking constantly. Reduce heat and simmer, whisking occasionally, 2 minutes, then whisk in salt, pepper, and nutmeg. Remove from heat and whisk in cheese, then cover pan.
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. Add spinach and sauté, stirring, until just wilted, about 3 minutes. Remove from heat and cool completely.
Stir together ricotta, egg, parsley, prosciutto, salt, pepper, and 1/3 cup cheese in a bowl, then stir in spinach mixture.
Preheat oven to 220°C.
Spread 2/3 cup sauce in buttered baking dish. Fill cannelloni with spinach mixture, and place in the dish. Spread ½ cup more sauce over cannelloni and sprinkle with remaining cheese. Bake, covered with foil, in middle of oven until sauce is bubbling, about 20 minutes.
Turn on broiler.
Remove foil and broil cannelloni about 5 inches from heat until lightly browned, 2 to 4 minutes. Let stand 5 minutes before serving. Reheat remaining sauce and serve on the side.