Preheat oven to 400 degrees Fahrenheit (200 degrees Celsius).

Slice zucchini in half lengthwise and use a small spoon to scoop out the flesh, leaving a 1/4-inch thick shell. Reserve the scooped flesh.

Finely chop the reserved zucchini flesh and set aside.

In a large skillet, heat olive oil over medium-high heat and add ground beef, onions, and garlic, cooking until beef is browned.

Incorporate the chopped zucchini flesh, oregano, salt, and pepper into the skillet and cook for 5 minutes until softened.

Remove skillet from heat and stir in the marinara sauce and breadcrumbs until thoroughly combined.

Arrange the zucchini shells in a 9x13 inch baking dish and fill each cavity generously with the beef mixture.

Top each filled boat with mozzarella and Parmesan cheese.

Bake for 20 to 25 minutes until the zucchini is tender-crisp and the cheese is golden and bubbling.

Remove from oven and let rest for 5 minutes before serving.