Preheat the oven to 390°F/200°C and line a baking pan with baking paper.
Cut the zucchinis lengthwise and scoop out the flesh with a spoon to create boats. (You can keep the zucchini flesh to make zucchini balls-find the recipe here)
Place the boats on the lined baking pan and set aside.
Finely chop the onion and heat the oil in a pan. Sauté the minced beef with the onion, seasoning with thyme, salt and pepper for 5-7 minutes.
Add the tomato sauce and mix well.
Divide the mixture into the boats and top with a layer of grated mozzarella cheese.
Bake for 30 minutes flipping the pan halfway through, until the zucchinis are cooked and the cheese gets golden brown. Serve hot.