1 12-oz. bottle Guinness Extra Stout
1 1⁄2 cups brown mustard seeds (10 oz.)
1 cup red wine vinegar
1 tbsp. kosher salt
1 tsp. freshly ground black pepper
1⁄4 tsp. ground cinnamon
1⁄4 tsp. ground cloves
1⁄4 tsp. ground nutmeg
1⁄4 tsp. ground allspice
1. Combine ingredients in a nonreactive mixing bowl. Cover with plastic wrap and let sit at room temperature for 1–2 days so that the mustard seeds soften and the flavors meld.
2. Transfer the mixture to the bowl of a food processor and process, stopping occasionally to scrape down the sides of the bowl with a rubber spatula, until the seeds are coarsely ground and the mixture thickens, about 3 minutes. Transfer to a jar and cover.
3. Refrigerate overnight and use immediately or refrigerate for up to 6 months. (The flavor of the mustard will mellow as the condiment ages.)