Melt the butter in a small saucepan, and add the flour. Whisk well for 30 seconds, then add the beer. Simmer for a minute or so, still whisking, and then add the cheese and mustard. Cook for another minute until the cheese has melted and the mixture is smooth. Season to taste.
Meanwhile, lightly toast the two slices of bread and lay them on a baking tray. When the cheese sauce is ready, spoon it onto the bread, gently pushing it to the edges with the back of the spoon (don't worry if it spills over the edges a bit!).
Place under the grill (broiler) for about 5 minutes, until the cheese sauce is just beginning to brown. Serve immediately.