Preheat your oven to 375°F (190°C).
Slice each zucchini in half lengthwise.
Use a spoon to scoop out the center, leaving a ¼-inch border.
Place the zucchini halves in a baking dish.
Heat olive oil in a large skillet over medium heat.
Add the ground beef and cook for 5–6 minutes until browned. Drain excess fat if needed.
Stir in the diced onion and garlic. Cook for another 2–3 minutes.
Add the cooked rice, tomato sauce, Italian seasoning, salt, and pepper. Stir to combine and let simmer for 3 minutes.
Spoon the beef and rice mixture into each zucchini half, pressing down gently to fill them completely.
Sprinkle Parmesan cheese on top if using.
Cover the baking dish with foil and bake for 20 minutes.
Remove the foil and bake for another 10 minutes until the zucchini is tender and the tops are slightly golden.
Let the stuffed zucchini cool for a few minutes before serving.