Preheat oven to 375°F and grease a baking sheet.
Cut 6 medium zucchini in half lengthwise, and scoop out the zucchini pulp, leaving the zucchini skins intact. Chop the zucchini pulp and set aside. Set the zucchini boats on the baking sheet.
Heat 1 tablespoon avocado oil in a large skillet over medium, and add 1 pound ground beef, crumbling as it cooks. Cook for 5-6 minutes until the ground beef is mostly browned.
Add ½ yellow onion and cook for an additional 1-2 minutes.
Stir in 1 ¼ cups marinara sauce, 1 tablespoon Italian seasoning, 1 teaspoon garlic powder, ¼ teaspoon salt, and zucchini pulp. Simmer over medium heat for 5-6 minutes.
Spoon the mixture carefully into the zucchini boats, then top with 2 cups shredded mozzarella cheese.
Bake for 25 minutes in the preheated oven, then sprinkle with ¼ cup fresh basil and serve!