Beer and Horseradish Mustard

from Real Beer and Good Eats

1/4 cup whole brown mustard seeds (can be found in the bulk spice section at Central Market)
1/4 cup whole yellow mustard seeds
1 cup lager
2/3 cup red wine vinegar
1 teaspoon ground allspice
1 teaspoon coarsely ground black pepper
1 teaspoon minced garlic
1 to 4 tablespoons prepared horseradish
2 teaspoons sugar
1 teaspoon salt
optional: 3 teaspoons dry mustard (used as a thickener)

Mix both mustard seeds, 1/2 cup of the lager, and the red wine vinegar in a small bowl, and let stand at room temperature for at least three hours.
Put the mixture in a blender, and add the remaining ingredients. Process to a coarse consistency. Scrape the mixture into the top of a double boiler. Stir the mustard over simmering water for 5-10 minutes until it begins to thicken, but is not quite as thick as commercial prepared mustard. Scrape the mustard into a jar, and let cool. It will thicken as it cools. If the mustard seems too thick after cooling, thin it down with a teaspoon or so of additional beer or vinegar. Cap the jar, and store the mustard in the refrigerature where it will last indefinitely.       Makes 2 cups.