Ingredients

• 8 slices firm white sandwich bread, crusts discarded
• 7 tablespoons unsalted butter, softened
• 1 lb mushrooms (preferably small), trimmed and halved
• 2 tablespoons all-purpose flour
• 1 1/2 cups whole milk
• 1/2 cup sour cream at room temperature
• 1/2 teaspoon salt
• 1/4 teaspoon black pepper
• 3/4 teaspoon Maggi or Bovril seasoning, or to taste
• 8 thin slices cooked ham (3 oz)

Instructions

1. Preheat oven to 350°F.
2. Arrange bread slices on a baking sheet and spread with 2 tablespoons butter. Toast in middle of oven until golden, about 10 minutes.
3. Heat 1 1/2 tablespoons butter in a 12-inch heavy skillet over moderately high heat until foam subsides, then sauté mushrooms in 2 batches until liquid from mushrooms is evaporated and mushrooms are browned, 6 to 8 minutes, adding 1 1/2 tablespoons butter for second batch. Transfer as cooked to a plate with a slotted spoon.
4. Melt remaining 2 tablespoons butter in skillet over moderate heat, then whisk in flour and cook roux, whisking constantly, 2 minutes. Add milk in a slow stream and bring to a boil, whisking constantly. Reduce heat and simmer, whisking occasionally, 5 minutes. Whisk in sour cream, salt, pepper, and Maggi seasoning, then stir in mushrooms. Cook over low heat, stirring, until heated through, about 2 minutes. (Do not let boil.)
5. Arrange a slice of ham on each toast and top with mushrooms and sauce.