If you have oyster water, pour it into a large skillet and bring it to a light simmer. (If not, use two cups vegetable stock, or water.) Reduce the water by about half. Add the oysters and let them cook until they just begin to plump up. Remove the oysters to a bowl and let them cool.
Add the milk to the oyster water in the pan and return to a simmer.
While that's going on, melt the butter over medium heat in another saucepan and add the flour. Stir to make a blond roux. Don't allow the mixture to brown.
Strain the oyster water-milk combination to the roux. Whisk the mixture until it takes on the texture of light mashed potatoes. This is bechamel.
Remove the bechamel from the heat and stir in the parsley, green onions, salt, lemon juice, Tabasco, and Worcestershire sauce. Drain the water from the bowl holding the oysters into the bechamel, and whisk in.
Add the crabmeat to the bowl of oysters. Toss gently to combine. Divide the oysters among eight gratin dishes or four small casseroles.
Spoon the bechamel over the oysters and crabmeat, and smooth it out. Top with a thin layer of bread crumbs.
Place the gratin dishes or casseroles on the top rack of the oven. Turn the oven on broil and raise the heat to 500 degrees. Broil until the sauce is bubbling and the bread crumbs are browning--15-20 minutes. Serve hot with hot French bread on the side.