Preheat your oven to 400°F and line a large baking sheet with parchment paper for easy cleanup.
Slice the zucchinis in half lengthwise and use a small spoon to carefully scoop out the center flesh, creating a hollow boat with walls about 1/4-inch thick.
Heat the extra virgin olive oil in a large skillet over medium heat, then add the finely diced onion and minced garlic, sautéing until fragrant and translucent.
Add the ground beef to the skillet, breaking it into small crumbles with a spatula, and cook until the meat is browned and no longer pink.
Stir in the tomato sauce, dried oregano, sea salt, and black pepper, allowing the mixture to simmer for 3 minutes so the flavors can fully develop.
Arrange the zucchini boats on the prepared baking sheet and fill each hollowed center generously with the savory beef mixture.
Sprinkle the shredded cheddar cheese evenly over the top of each boat.
Bake for 15 to 20 minutes, or until the zucchini is fork-tender and the cheese has melted into a golden, bubbly crust.