Step 1 Soak the mustard seeds: Place the mustard seeds and powder in a medium glass or ceramic bowl along with the cider vinegar and hard cider. Set aside, covered (not airtight), for 24 hours.
Step 2 Place the mixture in a food processor along with the salt and sugar, and process for 1 to 2 minutes until the seeds are coarsely ground. Add the chopped apple and pulse a few times to incorporate. This makes about 1 2/3 cups mustard.
Step 3The mustard will be very pungent at first. Cover and refrigerate for a few days (or to taste) before using.