Trim stems and remove any wilted outer leaves from Brussels sprouts; wash. Halve any large Brussels sprouts; set aside. For brine, in a very large bowl or deep container stir together the 8 cups cold water and 1/2 cup kosher salt until salt is completely dissolved. Add Brussels sprouts to brine mixture, making sure the sprouts are completely submerged (weigh down with a plate if necessary to keep sprouts submerged). Let stand at room temperature for 1 hour.
Drain Brussels sprouts; do not rinse. In a 12-inch cast-iron skillet or shallow roasting pan toss Brussels sprouts with olive oil to coat. Roast Brussels sprouts, uncovered, in a 350 degree F for 25 to 30 minutes or until tender, stirring once halfway through roasting.
Meanwhile, in a small skillet heat mustard seeds over medium-low heat about 5 minutes or until seeds are lightly toasted, shaking skillet occasionally. Remove seeds from skillet and crush seeds slightly. Add crushed seeds, the cracked black pepper, and, if desired, the 1/4 teaspoon kosher salt to cooked Brussels sprouts, tossing well. Serve immediately.