In a jar or glass container with an air-tight lid add the brown and yellow mustard seeds and the beer. Mix well, close the lid and refrigerate. Let seeds soak in the beer for a minimum of 12 hours, 24 recommended.
Take the soaked mustard seeds out of the refrigerator and set aside. In a non-reactive container mix the vinegar, salt and sugar and stir until all solids are dissolved.
Add the vinegar solution to the beer soaked mustard seeds. Mix well.
Transfer the mixture to a food processor and process until you reach the desired consistency. For whole grain, coarse mustard texture about 30 sec to 1 minute. For pasty, smoother consistency continue to process until satisfied.
Transfer the beer mustard to jars or glass containers with air tight lids and refrigerate. Flavors will continue to develop over time but you can consume the mustard as soon as a day after processing it.
The mustard will keep well refrigerated for up to three months.
Notes
*Brown mustard seeds are quite a bit spicier than yellow ones. If you are not a fan of hot mustard (even though it is a relative thing) consider using only yellow seeds.When planning to use this recipe budget for the needed resting time to allow the flavors to develop.For best results allow for 12 hours of soaking time for the first stage where the seeds absorb the moisture and flavors of the beer.Keep refrigerated.For tips, flavor twists and use ideas please consult the section of this post underneath the recipe card.If you end up with mustard that is too spicy for your liking you can tune the heat down by keeping it in a sealed glass container at room temperature for a few days (once satisfied refrigerate) or you could transfer it to a saucepan, warm it up over LOW heat (do not let it simmer) for 30-40 minutes and then transfer it back to a glass container and cool it down before you refrigerate it.