Bring a pot of lightly salted water to a boil. Once boiling, add the round zucchini and cook for just 5 minutes; they should be slightly parboiled, not fully soft.
Remove the zucchini from the water, slice off the top caps, and gently scoop out the interior flesh using a teaspoon, collecting the pulp in a bowl.
Heat 2 to 3 tablespoons of extra virgin olive oil in a skillet with the chopped onion. Roughly chop the scooped zucchini pulp with a knife and add it to the pan.
Sauté the zucchini pulp until tender, season lightly with salt, and set aside to cool.
In a mixing bowl, combine the cooked zucchini flesh, grated cheese, breadcrumbs, and the egg. Mix thoroughly until well combined.
Gently fold in the diced mozzarella.
Fill each hollowed zucchini with the stuffing mixture. Sprinkle the tops with a little extra breadcrumbs and drizzle generously with extra virgin olive oil.
Bake in a preheated oven at 180–200°C (350–400°F) for about 30 minutes, until a crisp, golden-brown crust forms on top.
Variations and Tips
You can add a few tablespoons of ricotta cheese to the filling to make the texture even creamier.