Line a baking tray with aluminum foil. Turn on your broiler to medium-high.
Melt the butter in a medium-sized pot over medium heat. Add the shallots and garlic and fry for a couple minutes or until soft and fragrant. Using a wooden spoon, mix in the mustard powder and flour. Let the mix cook and bubble for about a minute, stirring frequently. Add in the ale and Worcestershire sauce. Once evenly combined, add the grated cheese slowly so that it warms and melts. The mixture is quite dry and this is how it should be. Gradually add the cheese and continue to help it melt by stirring. Be careful not to let the mixture boil. You just want it warm enough to melt the cheese.
Once melted you should have a smooth, thick and almost elastic-like mixture. Turn the heat off and leave to cool slightly.
Toast your slices of bread and then place on the prepared baking tray. Divide the cheese mixture evenly among all pieces of bread, taking care to spread it out so it covers the entire top of the bread. Broil under high heat for 4 – 8 minutes or until the tops have colored to your liking.
Enjoy immediately with some arugula on the side. I think the peppery leaves are a great balance to the rich cheese topping.