Preheat the oven to 180C, 350F, Gas Mark 4 and grease a shallow ovenproof dish. Blanch the courgettes in boiling water for 4-5 minutes. Remove and cut in half lengthways. Scoop out the middle, chop and set aside.
Melt the butter in a saucepan, add the onions and garlic and fry gently for 3-4 minutes, until soft.
Stir in the courgette, 3/4 of the cheese, egg yolk, oregano and seasoning to taste. Divide into 8 and use to fill the courgette shells.
Arrange in the ovenproof dish, sprinkle with the remaining cheese and bake for 35 minutes, until tender and golden. Serve immediately.