If you're sick of your regular side of green beans, this casserole is sure to jazz up any holiday spread. You can premake the shallots the day before to save a little time on the day!
Preheat oven to 375ºF.
Blanch the beans in boiling salted water until tender. Shock in ice water and dry. Cut them into 1 ½ inch lengths.
Separate the shallot slices with your fingers. Toss shallot slices in a little flour, just enough to dust, then fry them in hot oil in batches until light golden brown. Drain on paper towels and set aside.
Melt half the butter in a saute pan until foamy. Add mushrooms, stirring over moderately high heat until they begin to release their liquid. Lower the heat and stir occasionally until the mushrooms are limp.
Deglaze with the sherry, cook for a minute to dry out a bit, then melt the remaining butter over medium heat. Add the flour and stir thoroughly. Cook for a minute or two then add the milk in small increments, stirring continuously. It will start to thicken up immediately. Keep stirring vigorously and evenly to smooth out the lumps. Add the cream and cook on a low heat for another 5-10 minutes to thicken. If the sauce is too thick, add a little milk or water. If it’s too thin, then cook down and reduce a bit. It should be the consistency of heavy cream. Season the mushrooms and sauce mixture with salt and white pepper.
Toss the mushroom mixture with the beans and put into a buttered glass or ceramic casserole dish. Scatter the shallots on top. Bake for15 minutes.