Preheat your oven to 200°C (390°F).

Slice each zucchini in half lengthwise and scoop out the flesh to create 'boats'. Sprinkle with salt and let sit for 10 minutes.

In a saucepan, heat olive oil and sauté minced garlic and chopped onion for 5-7 minutes. Stir in diced tomatoes, passata, oregano, basil, and sugar. Simmer for 6-8 minutes.

In a skillet, heat olive oil and sauté onion for 5 minutes. Add ground beef, seasoning with salt and pepper. Cook until browned, then add half of the sauce and simmer for 15 minutes.

Pat the zucchini halves dry, place cut-side up in a baking dish, pour the remaining sauce over them, fill with the beef mixture, and top with Gouda and Parmesan cheese.

Cover the dish with parchment paper and foil, then bake for 25 minutes. Remove the cover and bake for another 5-10 minutes until the cheese is melted.

Let cool for a few minutes before serving warm.