Prep the zucchini: Preheat the oven to 400°F. Halve each zucchini lengthwise and scoop out the seedy centers with a spoon to make boats, leaving about a 1/4-inch shell. Place them cut-side up in a baking dish, brush with the olive oil, and sprinkle lightly with salt.

Pre-bake the shells: Bake the empty zucchini boats for 10 minutes to drive off some moisture while you make the filling. This keeps them from turning watery later.

Brown the beef: Meanwhile, cook the ground beef and onion in a skillet over medium-high heat, breaking it up, for 6-7 minutes until no pink remains and the beef reaches 160°F. Drain off any excess grease.

Make the filling: Stir in the garlic and cook for 30 seconds, then add the marinara, Italian seasoning, salt, pepper, and red pepper flakes if using. Simmer for 3-4 minutes until thickened.

Fill the boats: Spoon the beef mixture evenly into the pre-baked zucchini shells, pressing it in gently and mounding it up.

Add cheese and bake: Top with the mozzarella and parmesan. Bake for 18-20 minutes, until the cheese is melted and bubbly and the zucchini is fork-tender.

Garnish and serve: Let them cool for a few minutes, then sprinkle with fresh basil or parsley and serve.