Open the oysters and tip into a small saucepan with their juice, keep the bottom of the shell.
Tip away most of the juice only keeping enough to cover the oysters in the pan. Bring to the boil then turn off the heat.
In another pan melt the butter and soften the onion. Add the flour and stir. Add the milk a little at a time to make a sauce, stirring all the time until thickened. Add salt and pepper and add some of the oyster juice to make a nice thick sauce.
Replace the oysters in shells, place on a baking tray and spoon over the sauce, sprinkle with bread crumbs and fish stock powder.
Bake in a hot oven for 10 to 15 minutes. When they start to bubble they are ready.