Smoked Salmon Croque Madames

Make the béchamel sauce: Melt 1 tablespoon butter in a wide medium saucepan over medium-high heat. Whisk in flour; cook, whisking, 30 seconds. Gradually whisk in milk; cook, whisking, until mixture comes to a boil and has thickened, about 2 minutes. Remove from heat. Season with salt and pepper.

Using remaining 4 tablespoons butter, butter 1 side of each piece of bread. Set aside 4 slices. Spread unbuttered sides of remaining 4 slices with mustard; top with salmon. Scatter dill over salmon. Top each with 1/4 cup cheese. Sandwich with remaining bread, buttered side up.

Preheat broiler. Heat a large nonstick skillet over medium-low heat. Add sandwiches; cook, turning once, until golden brown, about 2 minutes per side. Spread 2 tablespoons reserved béchamel sauce on top of each sandwich; sprinkle each with 1/4 cup remaining cheese. Broil until cheese is bubbling and golden brown.