Start by melting your butter, then add the flour and stir in for 30 seconds to cook out the flavour of the flour.
Slowly add ale and continue to stir.
Add cheese and mustard and stir till melted.
Slice and wash the green part of the leek and fry in a separate pan then add to the rarebit mix.
Toast some bread, spread a nice thick layer of rarebit covering all the corners of the bread and whack under the grill till done.
I love this recipe with truffle oil so give it a drizzle if you have some!