You will need
Preheat oven to 220C/425F/Gas 7.
Saute the onion in the butter until soft.
Add the mushrooms and cook for 2 minutes, stirring often.
Sprinkle over the flour and stir well until all the butter has been absorbed.
Add the milk, a little at a time, stirring continuously. Bring to the boil gently and continue stirring until the sauce has thickened. Simmer gently for 5 minutes. Season well.
Add the drained oysters and mix well. Remove from the heat.
Cut the avocados in half lengthways and remove the stones.
Pile the filling mixture into the centre of each half.
Mix the breadcrumbs and cheese together and spread thickly over the top, covering the flesh as well as the filling mixture.
Bake for 20-30 minutes.