Heat the oven to 200C/180C fan/gas 6. To make the pretzel croutons, tear the pretzels into small pieces and place on a roasting tray. Drizzle over the oil, then toss together to coat using your hands. Bake in the oven for 15-20 mins until golden and crisp.

Meanwhile, melt the butter in a medium saucepan over a medium heat. Once melted and foaming, stir in the flour to make a paste. Cook, stirring from time to time, for 2-3 mins, until slightly darkened. Mix in the garlic powder, mustard powder and cayenne pepper, if using, then gradually and slowly pour in the milk, splash by splash, whisking well after each addition. Pour in the beer and whisk it in. Bring to a simmer and leave to bubble for 5 mins until thickened and reduced slightly. Remove from the heat and whisk in the Worcestershire sauce and cheese. Pour into a warm dish and sprinkle with chives. Serve straightaway.