Preheat the oven to 375°F (190°C) and spray two 9 x 13-inch baking pans generously with oil.
Trim the ends and cut each zucchini in half lengthwise. Use a large spoon to scoop out the seeds and some flesh to form shells, then arrange the halves in the prepared baking pans, cut-side up.
Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add the ground beef and cook, breaking it up with a spatula, until no pink remains, about 3–5 minutes. Drain excess fat if needed.
Add the minced garlic and diced sweet onion to the skillet and cook 2–3 minutes until softened and fragrant.
Stir in 2 cups marinara sauce, 1 tablespoon Italian seasoning, 1 teaspoon balsamic vinegar, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Bring to a gentle boil, then reduce heat and simmer 2–3 minutes until the sauce is absorbed and thickened.
Spoon the meat mixture evenly into the prepared zucchini shells.
Top each stuffed zucchini with the shredded mozzarella cheese.
Bake in the preheated oven 20–25 minutes, until the zucchini is tender when pierced with a fork and the cheese is fully melted.
Remove from the oven and serve warm.