How to Make It
In a medium skillet, heat 1 tablespoon of the olive oil. Add 1 cup of the onions and cook over moderately high heat, stirring frequently, until golden. Add the oregano and sugar and season with salt and black pepper. Cook, stirring, for 1 minute. Add the wine and simmer, stirring, for 1 minute. Add the tomato sauce and water and simmer over moderate heat until slightly thickened, 10 minutes. Remove from the heat.
Melt the butter in a medium sauce-pan. Add 1/4 cup of the onions and cook over moderate heat, stirring, until softened, about 4 minutes. Whisk in the flour until combined. Add the milk in 2 batches, whisking constantly until smooth before adding more. Bring to a boil over moderately high heat, then simmer over moderate heat until thickened, about 2 minutes. Whisk in 1/4 cup of the Parmesan cheese, the nutmeg, white pepper and 3/4 teaspoon of salt. Remove from the heat and press a piece of plastic wrap directly on the surface of the white sauce.
In a large skillet over high heat, add the spinach by handfuls and toss until wilted. Wipe out the skillet. Squeeze the spinach dry, then finely chop.
In the same skillet, heat the remaining 1 tablespoon of olive oil. Add the bacon and cook over moderately high heat until golden, about 3 minutes. Add the garlic and cook, stirring, until fragrant, about 30 seconds. Add the remaining 1/4 cup of chopped onion and cook until translucent, about 3 minutes. Add the spinach and cook, stirring, for 2 minutes. Add 1 cup of the white sauce and 1 teaspoon each of salt and black pepper. Remove from the heat and stir in 1/2 cup of the remaining Parmesan cheese and the chopped walnuts.
Preheat the oven to 400°. Lay 16 crêpes on a work surface and spread 3 tablespoons of the spinach filling in a strip down the center of each one. Tightly roll the crêpe around the filling, tucking in the edges as you roll to form a neat cylinder. Repeat with the remaining filling and crêpes.
Spread 1/4 cup of tomato sauce on the bottom of a 9-by-13-inch glass or ceramic baking dish. Arrange the cannelloni over the tomato sauce. Spread the remaining white sauce over the cannelloni. Drizzle the remaining tomato sauce on top and sprinkle with the remaining 1/4 cup of Parmesan. Bake for 25 minutes, or until the cheese is golden and the cannelloni are heated through. Let stand for 10 minutes before serving.