Preheat your oven to 400°F (200°C). Wash the zucchinis and slice them in half lengthwise. Using a small spoon, gently scoop out the center flesh, leaving about a 1/4-inch border around the edges to form boats. Be careful not to puncture the skin. Finely chop the scooped-out zucchini flesh and set it aside. Place the hollowed-out boats cut-side up on a baking sheet lined with parchment paper. Brush them lightly with olive oil and sprinkle with a pinch of salt and pepper. Bake for 10 minutes to soften slightly and reduce moisture.

While the zucchini bakes, heat a large skillet over medium-high heat. Add the ground beef and cook, breaking it up with a spoon, until browned and no pink remains, about 5-7 minutes. Drain any excess fat from the skillet.

Add the diced onion to the skillet with the beef. Cook for 5 minutes until softened. Stir in the minced garlic and tomato paste, cooking for 1 minute until fragrant.

Add the chopped reserved zucchini flesh to the skillet. Sauté for 5-7 minutes until the moisture cooks out.

Stir in the marinara sauce, Italian seasoning, salt, black pepper, and red pepper flakes (if using). If adding rice or cauliflower rice, mix it in now. Simmer for 5 minutes to let the flavors combine.

Remove the partially baked zucchini boats from the oven. Spoon the ground beef filling into each boat, mounding it slightly over the top.

Sprinkle the tops evenly with shredded mozzarella and grated Parmesan cheese.

Return to the oven and bake for 15-20 minutes until the zucchini is fork-tender and the cheese is melted and golden brown.

Let cool for a few minutes, then garnish with fresh parsley or basil before serving.