Preheat oven to 400 degrees.
Slice the zucchini in half lengthwise and use a spoon to scoop out the middle of the vegetable. Leave about ½ inch along the outside of the zucchini creating a “boat.” NOTE: If you prefer the zucchini to be soft when serving, bake the zucchini boat for 5 to 10 minutes before stuffing.
Cook the ground beef and onions together in a large skillet until the beef has completely browned. Drain well.
In a large bowl, combine the ground beef, onions, black beans, taco seasoning, and 1 cup of cheese.
Spoon the beef mixture into the zucchini boats, pushing it down. Pile the extra taco mixture on top until the boats are slightly overflowing. Top with the remaining cheese.
Bake Zucchini Boats for 15 to 20 minutes until the cheese is melted and the zucchini has softened a bit.
Garnish lime wedges, seeded and diced plum tomato, diced avocado, chopped cilantro, and sliced black olives.