Preheat the oven to 350°F
Heat a large pan of water. When it reaches the boil, add the zucchini and cook for 5 minutes. Drain and cool for a couple of minutes then slice them in half lengthwise.
Carefully scoop out the pulp into a mixing bowl, making sure not to break the shell of the zucchini.
Melt the butter in a frying pan and fry the onion until just softened. Add the beef and cook gently for one minute. Add the prosciutto and continue cooking until the meat is thoroughly browned.
Drain all the cooking fat from the pan and add the meat and onion mixture to the zucchini pulp.
Stir in the eggs, breadcrumbs, parmesan and basil. Season with salt and pepper. Mix well.
Spoon the mixture into the zucchini halves.
Pour the tomato sauce into a large baking dish that has been lightly greased with olive oil.
Carefully place the zucchini into the dish and bake for 15-20 minutes, or until the filling is browned but not overdone.
Serve hot, warm or at room temperature with a bit of tomato sauce from the dish.