Yes, you can make mustard. it’s easy. Try this recipe from the Los Angeles Times. And if you don’t have both brown and black mustard seeds, don’t worry. Just use one or the other. It’s just prettier with both.
About 1/4 cup plus 3 tablespoons (2 1/2 ounces) brown mustard seeds Scant 1/4 cup (1 1/4 ounces) black mustard seeds About 1/4 cup plus 2 tablespoons (1 1/4 ounces) mustard powder 1/2 cup cider vinegar 3/4 cup flat hard apple cider 1 1/2 teaspoons kosher salt 2 1/2 tablespoons sugar 1 apple, peeled, cored and finely chopped
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 (but not sealed airtight), for 24 hours.
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.
The mustard will be very pungent at first. Cover and refrigerate for a few days (or to taste) before using.