Heat the oven to 400°F. Halve zucchini lengthwise and scoop out the centers, leaving 1/4-inch walls; finely chop the removed flesh. Brush boats with 1 teaspoon oil, season with 1/4 teaspoon salt and bake cut-side up on a rimmed sheet pan for 10 minutes.
While the zucchini bakes, heat 1 teaspoon oil in a skillet and brown beef and onion for 7 to 8 minutes until the meat reaches 160°F. Drain excess fat.
Add chopped zucchini flesh and cook for 5 to 6 minutes until its released water evaporates. Stir in garlic, tomato sauce, oregano, remaining salt and pepper and simmer for 2 minutes.
Fill the softened boats evenly. Mix panko, Parmesan and remaining 1 teaspoon oil and scatter over the filling.
Bake for 15 to 18 minutes until zucchini is tender, crumbs are golden and the filling reaches 165°F. Rest for 5 minutes and serve two boats per person.