Preheat the oven to 375°F (190°C) and lightly grease a baking dish.
Slice each zucchini in half lengthwise. Scoop out the centers with a spoon, leaving about ¼ inch of zucchini around the edges to create sturdy boats.
Brush the zucchini with olive oil and season lightly with salt and pepper. Arrange them in the prepared baking dish.
Heat the remaining olive oil in a large skillet over medium heat.
Add the onion and cook for 3–4 minutes until softened. Stir in the garlic and cook for 30 seconds.
Add the ground beef and cook until browned, breaking it into crumbles. Drain excess grease if needed.
Stir in the taco seasoning, tomato sauce, diced tomatoes, and black beans (if using). Simmer for 5 minutes.
Spoon the taco mixture evenly into each zucchini boat.
Sprinkle the shredded cheese over the top.
Bake for 20–25 minutes, or until the zucchini is tender and the cheese is melted and bubbly.
Garnish with your favorite toppings and serve immediately.