Slice each zucchini in half lengthwise and score the flesh without cutting through the skin, then scoop out the flesh with a spoon or melon baller and finely chop it; pat the shells dry.

Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper or lightly oil it.

Heat 2 tablespoons of olive oil in a large skillet over medium-high heat and add the ground beef; cook for 5 to 7 minutes until browned, breaking it apart, then drain excess fat.

Add the diced onion to the skillet and sauté for 2 to 3 minutes until softened, then stir in the minced garlic and cook for 1 more minute; remove from heat.

Transfer the meat mixture to a mixing bowl and stir in the chopped zucchini flesh, ricotta cheese, ¾ cup of the mozzarella, Parmesan, basil, Italian seasoning, red pepper flakes, salt, pepper, and marinara sauce until evenly combined.

Arrange the zucchini shells on the prepared baking sheet, skin-side down, and divide the filling evenly among them, mounding slightly.

In a small bowl, combine the remaining ¼ cup mozzarella with the breadcrumbs and sprinkle over the stuffed zucchini boats.

Bake in the preheated oven for 30 to 35 minutes, or until the zucchini is tender and the topping is golden brown; cover with foil if browning too quickly.

Let the stuffed zucchini rest for 5 minutes before serving to allow the filling to set.