Preheat your oven to 375°F (190°C). Gather a baking dish, skillet, and mixing bowl.

Wash the zucchinis, cut each one in half lengthwise, and scoop out the seeds and flesh, leaving a 1/4-inch thick shell.

Brush the baking dish with olive oil and arrange zucchini halves cut side up.

In a skillet, heat a tablespoon of olive oil over medium-high heat. Add ground beef, cooking for 5-7 minutes until browned.

Add chopped onion and minced garlic to the skillet. Sauté for 2-3 minutes until onion is translucent.

Sprinkle salt, black pepper, and dried oregano into the mixture, stirring well to combine.

Add diced tomato and stir for another 2-3 minutes. Remove from heat and let cool slightly.

In a mixing bowl, combine half of the mozzarella cheese, all of the Parmesan, and the beaten egg with the cooled beef mixture.

Fill each zucchini half with the mixture, topping with remaining mozzarella cheese.

Bake for 25-30 minutes until zucchini is tender and cheese is melted and golden.

Allow to cool for about 5 minutes before serving.