Slice the zucchini lengthwise as fine as possible. If we have a mandolin, the task will be sewing and singing. If this is not the case, with a sharp knife and patience, the thing is also achieved. We select the 12 central sheets and cut them in half.
We are going to use the rest of the zucchini for the filling, so we chop it finely and do the same with the onion. Heat a little oil in a frying pan and fry the onion. over low heat until translucent. Add the chopped zucchini and continue frying for about ten more minutes.
Chop the slices of ham and add them to the pan along with the cream cheese. Stir until a homogeneous mass is obtained let cool slightly before assembling the cannelloni.
We extend a sheet of transparent paper and place four overlapping sheets of zucchini on top. We put a filler cord on one of the ends and wind cylinder shaped helping us with the paper. Repeat the operation with the rest of the zucchini slices.
We place them on a baking sheet covered with parchment paper and cover them with a little tomato sauce. Then sprinkle grated cheese on top and we gratin in the oven for three to four minutes or until the cheese begins to color. We serve immediately.