How to cook
1. Cut off both ends of the zucchini and cut it into sheets as thin as possible. This is best done with a slicer but you can do it by hand if you don't have one (it's also best to have a zucchini that's straight, not a curvy one, to make things easier).
2. Bring water to a boil (enough to cover the zucchini) with 2 pinches salt, 1 pinch sugar and the vinegar. Put the zucchini in it and parboil for 3-4 min., then take them out and strain. Leave them to cool on paper towels to further drain away any remaining water.
4. Saute the vegetables in the heated butter and olive oil and sprinkle with a pinch of salt. Once softened and all the liquid gets absorbed until they're left in just oil, pour in the flour all at once. Stir vigorously for 1-2 min. until it crisps slightly and starts smelling nice (be sure to cook on medium heat).
5. Pour in a little of the milk and stir - it's going to turn into a sludge right away. Pour in the rest in several lots, while stirring. Season with the spices and salt and cook another 5-8 min. while stirring nonstop. You're going to get a thick Bechamel that's going to serve as your filling. Let it cool and thicken some more.
6. You're going to need transparent foil to assemble the Cannelloni. Lay out a large sheet of it on the counter and arrange the thin zucchini sheets, overlapping them, until you run out. See the photos I've provided.
7. Put the filling in the middle, even it out and roll it up by lifting the foil first from one side, then the other. It's quick and easy.
8. Remove the foil and cut it into 4 Cannelloni. Serve them as a cold appetizer, sprinkled with chopped parsley, a little salt and olive oil.