Pour the mustard seeds, vinegar, and beer into a bowl. Whisk together and allow to soak 8 hours or overnight. Stir in the sugar, honey, salt, and mustard powder. Place about ⅔ of the mixture in a food processor and grind until it becomes a paste, then pour back into the bowl with the unprocessed amount and mix well. Pack the mustard into a pint-sized mason jar and allow to rest for a day in the fridge before using. Keep refrigerated.