Sprinkle a little salt and pepper on the foie gras slices, and sear them in a large skillet over medium-high heat, until lightly browned. Remove the foie gras and keep warm.
In the same skillet, bring one stick of the butter to a bubble and add the mushrooms. Cook until the mushrooms become tender.
Meanwhile, make a thin béchamel. Heat the milk until warm but not steaming (the microwave works perfectly). In a saucepan, heat the remaining half-stick of butter until it bubbles. Sprinkle in the flour and stir in until it turns into a roux. Don't let it brown. Lower the heat and whisk in the milk slowly and constantly until a thick sauce forms.
Add the sherry to the mushrooms and bring to a boil until the alcohol has evaporated--about two minutes. Lower the heat to a simmer, and stir in the béchamel. Add salt and pepper to taste, and cook over very low heat, stirring lightly, until the sauce penetrates the mushrooms.
Place a slice of foie gras atop the slices of toast on plates. Pour about a half-cup of the mushroom mixture over the toast. Serve hot.