Place all of the ingredients in a sterilised jar and mix well. Leave to soak in the fridge for at least 48 hours. The seeds will soak up the liquid and expand, so make sure the container is big enough to allow for this
Transfer to a blender and blitz to a smooth paste. If you'd prefer a coarse mustard, pulse the blender so some of the seeds stay whole. If you want a smoother finish, pass the mustard through a fine sieve after blending
Taste and season if needed. This recipe packs a little punch at first but the heat will fade after a while. If you want to liven up the mustard again after a few weeks, stir through some more vinegar
Alternatively, if you’d like it milder in flavour transfer to a small saucepan and gently simmer for 5–10 minutes, adding a little vinegar or water if it needs loosening
