Preparation

• Position a rack 4 to 5 inches from the broiler and heat the broiler on high.
• Lightly butter a large, rimmed baking sheet. Smear one side of each slice of bread with the mustard. Set the bread slices mustard side up on the baking sheet and top with the turkey.
• Melt the butter in a 2-quart saucepan over medium heat and add the scallions. Cook for 1 minute, stirring often. Whisk in the flour and cook for 1 minute more, stirring often. Add the milk and beer; whisk until thick and bubbling, about 2 minutes. Add all but 1/4 cup of the cheese, the Worcestershire, and 1/2 tsp. pepper and whisk until bubbling, just a few seconds. Season to taste with salt and pepper.
• Spoon 1/4 cup of the cheese sauce over each sandwich. Sprinkle with the remaining cheese.
• Broil until bubbling and browned, 4 to 5 minutes. Cool for a couple of minutes before serving.

Rarebit (also called Welsh rarebit) is a popular British dish that typically consists of a mixture of melted Cheddar, beer, seasonings like Worcestershire sauce and mustard, and sometimes milk, served over toast.