Saute mushrooms in butter until they are soft and most of the liquid has evaporated about 5 minutes.
Sprinkle flour over mushrooms and stir with a whisk to blend. Slowly stir in milk in 4 separate parts, whisking each addition well to ensure no lumps are left; add salt & pepper. Cook until sauce is smooth and slightly thickened. Place cauliflower is a buttered casserole, pour sauce over top. Combine cheese and bread crumbs and sprinkle over top.
Bake for 15 to 20 minutes or until top in browned, and cheese is melted.