Prepare the vegetables for stuffing by removing the stalks of 4 of the mushrooms (save the stalks for the stuffing). Halve the tomatoes and discard the seeds. Halve 2 of the zucchini lengthwise and scoop out the centers with a spoon (save the centers for the stuffing).
Arrange the prepared vegetables on a greased baking sheet, season lightly with salt and pepper.
Mince or grate the remaining 3 mushrooms and the leftover stems. Dice the remaining zucchini.
Saute the ground pork (season with your favorite seasonings) with the onions over medium heat and add the minced mushrooms. Then add the diced zucchini plus the remaining centers.
Cook until soft, then add the can of tomatoes and continue to cook until very little liquid is left, about 10 minutes.
Pile the filling into the prepared vegetables on the baking sheet and sprinkle with mozzarella cheese. Bake for 35-45 minutes.