Melt half the fat in a medium saucepan, then fry the bacon, if using, on a medium heat until golden and crisp. Scoop out using a slotted spoon, and set aside.
Add the onion and carrot to the hot fat in the pan and fry, stirring regularly, until they’re beginning to soften.
Stir in the sliced leek and continue until wilted.
Add the caraway seeds and paprika, fry for another minute, then pour in half the beer and bring to a simmer.
Leave to bubble away for about five minutes, until reduced by roughly half, then add the stock and bring to a simmer.
Meanwhile, melt the remaining fat in a small pan on a medium-low heat, then stir in the flour to make a paste.
Fry for a couple of minutes, until golden, then stir in the mustard powder.
Gradually whisk in the milk, until you have a smooth liquid.
Stir the milk mixture into the soup, add the remaining beer and bring to a simmer.
Cook on a medium-low heat, stirring regularly, until the soup thickens.
Turn the heat right down and add the cheese a little at a time, stirring in each batch until it’s melted before adding the next.
Taste and adjust the seasoning.
Divide the soup between bowls (or hollowed-out bread rolls), sprinkle with chopped chives and the reserved bacon, and serve.
