Grainy Porter Mustard - Garden Therapy gardentherapy.ca/grPrivate View Make it! In a small bowl, combine the yellow and brown mustard seeds, 1⁄4 cup vinegar, and the beer. Cover tightly with plastic wrap, and let sit at room temperature for at least 3 days and up to a week to hydrate the seeds and allow the flavors to mellow. Transfer the mustard mixture to a small food processor and process until mostly smooth ...