Prepare the duck breast: remove the skin and mince the meat with a knife (or use an electric chopper if you have one, but avoid chopping it too finely).
Render the fat: Cook the duck skin over low heat in a pan to render the fat, which will be used for cooking.
Prepare the zucchinis: cut the zucchinis in half lengthwise and scoop out the flesh, setting it aside (if using round zucchinis, refer to the technique in this article).
Cook the filling: peel and chop the onion, then sauté it in some duck fat. Season with salt, pepper, and your choice of spices. Add the chopped mushrooms.
Add the zucchini flesh: chop the zucchini flesh and add it to the pan. Cook until the liquid released by the zucchinis evaporates.
Add the duck breast, cook quickly over high heat for about 3 minutes.
Stuff the zucchinis: fill the zucchini halves with the mixture.
Sprinkle with Parmesan and breadcrumbs, then drizzle with olive oil.
Bake: place the stuffed zucchinis in the oven and bake for 20 minutes at 200°C (392°F). Serve hot.