Preheat your oven to 375°F.
Cut the zucchinis in half lengthwise and scoop out the seeds to create boats. Drizzle the insides with olive oil and sprinkle with salt and pepper. Place them cut-side up on a baking sheet.
In a large skillet over medium heat, add the ground beef, diced onion, and minced garlic. Cook until the beef is browned and the onion is translucent, about 5-7 minutes. Drain any excess fat.
Stir in the diced tomatoes, Italian seasoning, salt, and black pepper. Let the mixture simmer for about 5 minutes.
Spoon the beef mixture evenly into the zucchini boats. Top each boat with shredded mozzarella cheese.
Bake in the preheated oven for 20-25 minutes, until the zucchini is tender and the cheese is bubbly and golden.
Let the zucchini boats cool for a few minutes before serving. Garnish with fresh basil if desired.