Wash the zucchini, pre-cook them whole in a wide saucepan with enough water for about 10 minutes.
Remove and allow to cool slightly. Cut in half lengthwise and carefully scoop out to a 1/2 cm thick edge. Finely chop the flesh.
Peel and finely dice the onion. Heat the oil in a frying pan and sauté the onion.
Peel and press the garlic and add it. Mix in the mince and fry until crumbly, then fold in the diced zucchini pulp. Season with chopped herbs, salt and pepper and saute lightly for 10 minutes.
Remove from heat, fold in egg as well as cheese.
Meanwhile, for the sauce, melt the butter in a saucepan and sweat the flour in it until golden.
Gradually add the milk, stirring all the time, and stir to make a creamy sauce. Season with nutmeg, pepper, salt and juice of one lemon, remove from heat. Fold in the eggs and cheese, do not boil the sauce anymore!
Preheat the oven to 200 °C.
Place the zucchini halves side by side in a large gratin dish and fill with the meat-zucchini mixture.
Pour the sauce over the top and cook in the oven for half an hour.