Using the side of a chef’s knife, a rolling pin, or a small spice grinder coarsely crack peppercorns and mustard seeds. In a small mixing bowl, combine brown sugar, thyme, peppercorns, mustard seeds and salt. Liberally apply rub to roast. (If desired, the roast could be prepared to this point one day ahead and held, wrapped in food film and refrigerated.) Allow the roast to come to room temperature before cooking.