FROM A CHEF'S KITCHEN
Cut zucchini in half lengthwise. Scoop the seedy center out, creating a trench. Place on a parchment paper-lined baking sheet. Brush each side with olive oil and season. Roast for 10-12 minutes at 400 degrees.
Cook onion 4-5 minutes over medium heat. Add the ground beef and cook 6-7 minutes, breaking the meat up as you go.
Add garlic, tomato paste, water, dried basil, oregano and breadcrumbs. Cook 3-4 minutes. Stir in parsley and Parmesan cheese. Fill each zucchini "trench" with even amounts of the filling.
Simmer onion, garlic, tomatoes, tomato sauce, oregano and red pepper flakes for 15 minutes.
Spoon ⅔ of the tomato sauce into the bottom of a baking dish. Add the filled zucchini, top with the remaining marinara and sprinkle with Parmesan cheese. Cover with aluminum foil and bake for 35-40 minutes.