Slice and scoop: Cut each zucchini in half lengthwise, then use a small spoon or melon baller to scoop out the flesh, leaving about 1/4 inch of skin on all sides to keep the boat sturdy, about 3 to 4 minutes total.
Season the shells: Lightly brush or spray the inside and outside of each zucchini boat with olive oil, then season generously with salt, pepper, and a pinch of garlic powder.
Arrange on the baking sheet: Place the zucchini boats skin-side down on a parchment-lined baking sheet, ready for filling.
Brown the meat: Heat a large skillet over medium-high heat and add the ground turkey or beef, breaking it apart with a wooden spoon as it cooks for 5 to 7 minutes until no pink remains.
Add the vegetables: Drain excess fat if needed, then stir in the diced onion and bell pepper, cooking for another 3 to 4 minutes until softened.
Season the filling: Add the taco seasoning packet and chunky salsa, stirring well to combine, then add the black beans if using and simmer for 2 to 3 minutes to blend the flavors.
Taste and adjust: Sample the filling and add extra salt, pepper, or garlic powder as needed.
Fill the boats: Spoon the warm taco filling evenly into each zucchini boat, dividing it as equally as possible among the four boats.
Top with cheese: Generously sprinkle shredded cheese over each boat, using about 1/4 cup per boat, and press it down slightly so it holds.
Bake until golden: Transfer the baking sheet to a preheated 375 degrees Fahrenheit oven and bake for 15 to 18 minutes until the zucchini is fork-tender and the cheese is melted and lightly browned on top.
Garnish and serve: Remove from the oven and let cool for 2 minutes, then top with cilantro, sour cream, avocado, or other desired toppings before serving.