Bring the water, butter, salt and Brussels sprouts to a boil in a Dutch oven or a large deep skillet. Cover and steam over medium-heat until the Brussels sprouts are brightly colored and just tender, 5 to 10 minutes.
Remove the lid and continue to cook until the liquid evaporates, 1 to 2 minutes longer. Saute to intensify flavors, 1 to 2 minutes longer. Adjust seasonings, including pepper to taste, and serve.
Remove Brussels sprouts from pan. Add mustard to remaining liquid in pan. Stir and make sauce.