Bring a large pot of salted water to a boil. Add the penne and cook for half the time indicated on the package. Drain and let cool.

In a skillet, sauté the onion in olive oil, then add the zucchini. Season with salt and cook until tender. If needed, add a little pasta cooking water. Remove from heat and let cool.

In a large bowl, beat the egg. Add most of the grated cheese (reserve some for topping), mozzarella, black pepper, a pinch of salt, and chopped basil.

Add the pasta and zucchini to the bowl. Mix well and transfer to a baking dish lightly greased with olive oil. Sprinkle with the reserved cheese, black pepper, and breadcrumbs.

Bake in a preheated oven at 400°F (200°C) for 30 minutes, or until golden brown. Let cool slightly before serving.