Melt 25g of unsalted butter in a saucepan over medium heat until foaming. Stir in 25g of all-purpose flour and cook for 1 minute, then gradually whisk in 120ml of warm brown ale until thickened.
Stir in 140g of grated sharp cheddar until melted, then 1 heaping teaspoon of English mustard powder, 1 tablespoon of Worcestershire sauce, and 1 tablespoon of chopped chives.
Heat the broiler. Toast 2 large slices of sourdough bread on one side, turn, and spread the cheese mixture over the untoasted side. Broil for 1-2 minutes, until golden and bubbling.
