Place all of the seeds in a large bowl. Pour over the beer, stir, cover with clingfilm and leave at room temperature overnight.
The next day, tip the mustard seeds through a sieve over a bowl. Put half in a blender and blitz to break down the mixture, then return to the bowl along with the whole seeds.
Add the sea salt, malt vinegar and honey. Stir thoroughly and spoon
into sterilised jars.