Combine the dried porcini mushrooms and boiling water in a bowl; cover and let stand for 15 minutes. Drain in a sieve over a bowl, reserving mushroom liquid. Finely chop porcini mushrooms; set aside.
Heat 1 tablespoon oil in a large cast-iron or heavy skillet over high heat. Add chopped button mushrooms; cook for 5 minutes or until browned, stirring occasionally. Reduce heat to medium. Add porcini mushrooms, wine, pepper, and garlic; cook 1 minute or until liquid almost evaporates. Remove from heat.
Heat 1 tablespoon oil in a large sauce-pan over medium heat. Add onion; cook 3 minutes. Add reserved mushroom liquid, 2 1/4 cups water, porcini mixture, barley, and salt, and bring to a boil. Cover, reduce heat, and simmer 35 minutes or until barley is tender. Fluff with a fork; stir in parsley.
Carefully melt butter in a cast-iron or heavy skillet over high heat. Add the sliced button mushrooms, and sauté for 5 minutes or until browned. Top the pilaf with sautéed mushrooms.