Coat bottom of a medium pan over medium heat with olive oil. Add onions and cook until translucent, about 4 minutes. Add garlic and cook until translucent and fragrant, about 3 minutes more. Add the sausage and cook, stirring frequently and breaking up the sausage into crumbles, until cooked through, about 5 minutes. Remove mixture from the heat and allow to cool slightly. Add oregano and Parmesan and stir to combine.
Fill each zucchini with the sausage mixture, packing them slightly, and place on a sheet tray. Drizzle with olive oil and bake until golden brown, about 5 minutes.
This recipe has been updated and may differ from what was originally published or broadcast.