Here's the recipe for the Beer Mustard we served at the Slow Foods Redlands-Hangar 24 Event

Beer Mustard

Ingredients
4 tblsp yellow mustard seeds
1 tblsp brown or black mustard seeds
125ml red or dark ale (I used Smithwick’s [3])
75ml cider vinegar
1 small onion (about 100g), finely chopped
1 clove garlic, crushed
1 tblsp honey
1 tblsp dark muscovado sugar (or other dark brown sugar)
0.25 tsp turmeric
0.5 tsp allspice
0.5 tsp fine salt
You’ll also need:
A spice or coffee grinder to grind the mustard seeds and a blender or food processor to blend the mustard fixin’s, plus a jar or jars with non-corrosive lids, sufficient to store about 250g mustard.

Grind half of the yellow mustard seeds to a fine powder. Mix the powder together with the remaining whole yellow mustard seeds and the brown or black mustard seeds in a non-metallic bowl and set aside.
In a small, non-reactive saucepan, mix together the beer, cider vinegar, onion, garlic, honey, sugar, turmeric, allspice and salt. Bring to a boil, then simmer gently, uncovered, for about 10-15 minutes, until it reduces by about half.
Remove from the heat and pour the beer/vinegar mixture over the ground and whole mustard seeds. Leave to stand, covered but unrefrigerated, for about 48 hours. Do have a sniff occasionally during this period – ’tis lovely.
When ready for the last step, prepare your jars. Turn your oven on to 140C. Wash the jars in hot soapy water, rinse thoroughly, and sterilise, either by boiling in water for 10 minutes and then drying in the oven or just by keeping the jars in the oven for at least 30 minutes before using. Allow the jars to cool before filling.
Grind the mustard mixture in a food processor for 3-4 minutes, until the mixture is creamy but the seeds remain coarse and grainy. If the mixture is too thick, add a little more beer or vinegar.
Bottle the mustard, seal tightly with lids that have been dipped in boiling water, and, ideally, allow it to age for another 2-3 days in the fridge before using. It should keep for several months but it will lose flavour over time, especially once opened, so it’s best to keep it refrigerated.
The Variations:
You can vary the proportion of yellow and brown or black mustard seeds in the recipe according to your tastes – yellow seeds are mild, brown seeds are hot and aromatic, black seeds are strong and pungent.