Make the bechamel by heating the butter in a saucepan until it bubbles. Stir in the flour to make a blond roux; don't allow it to brown. Whisk in the milk over low heat until the sauce thickens. Add the salt and keep warm.
In a second saucepan, heat the butter until it bubbles, and in it saute the green onions, mushrooms, garlic and cayenne until the mushrooms are tender. Add the sherry and bring to a boil. Lower the heat and cook until all the liquid is absorbed.
Carefully add the lump crabmeat, agitating the pan to combine with the rest of the ingredients so as not to break the lumps. Spoon the mixture onto an oven-proof serving dish.
Nap two or three tablespoons of the bechamel over the top of the crabmeat. Put the plates into the hot broiler for about a minute. Serve immediately, warning that the plates are hot.