Preheat the oven to 350 degrees F. Line a sheet tray with heavy-duty foil.
Use a spoon to remove the pulp from the center of each zucchini half to create a boat, then add the pulp to a medium mixing bowl. Combine the pulp with the pork, veal, Parmesan, parsley, crushed red pepper, egg yolk, onions and some salt and pepper.
Place the zucchini halves on the prepared sheet tray hollowed-side up, then drizzle lightly with olive oil. Fill with the stuffing and top with the lemon zest. Bake until the zucchini are tender, about 45 minutes.