Mix 1/4 cup water, the oil and salt in a 12-inch skillet, and place Brussels sprouts, cut side down.  Mix mustards and remaining 1/4 cup water. Turn burner on high heat, place lid over skillet, and heat until water starts to steam. Continue to steam until water has evaporated, 4 to 5 minutes. Remove lid and continue to cook until Brussels sprout bottoms are impressively brown, a couple of minutes longer. Turn Brussels sprouts onto a wide serving dish and return skillet to the burner. Add mustard mixture and cook until heated through, almost instantly. Pour mustard over Brussels sprouts and season with pepper; toss to coat. Serve.