Prep Your Spices. Gather your mustard seeds, coriander seeds, peppercorns, allspice berries, bay leaves, red pepper flakes, cloves, salt, and brown sugar. Lightly crush the seeds (mustard, coriander, peppercorns, allspice) in a mortar and pestle or using a rolling pin.
Mix the Seasoning. In a small bowl, combine all the crushed spices with the bay leaves, red pepper flakes, salt, and brown sugar (if using). Stir or shake to ensure even distribution of flavors.
Store or Use Immediately. Use right away to season your beef brisket. Or store the seasoning in an airtight container in a cool, dry place for up to 6 months.