Crumble or chop dried mushrooms into small pieces, and place in a small bowl with hot water; set aside for at least 30 minutes.
Melt 2 tablespoons of butter in a wide, deep skillet over medium heat. Add ¼ cup of the onions, and sauté 3 minutes. Add fresh mushrooms, and cook briefly, just until softened, about 4 minutes. Transfer the mushrooms to a container; cover and keep warm.
In a medium saucepot, heat broth to boiling. Cover, and reduce heat to low, holding at a simmer.
In the same skillet that was used for the mushrooms, melt the remaining 2 tablespoons of butter over medium heat. Add the remaining ¾ cup onions, and sauté until translucent, about 4 minutes. Stir in rice, and cook for 2 minutes. Add wine, dried mushrooms with their liquid, and salt; simmer until all liquid is completely absorbed.
Begin adding 3 cups of hot broth in 1-cup increments, stirring constantly until each addition is absorbed before adding the next one. (This should take 15 to 18 minutes total.) Add the remaining broth in ½-cup increments, stirring constantly. Test the risotto between additions until it’s done cooking.
Remove the risotto from the heat, stir in pecorino, and divide among four serving bowls. Top each with a portion of the reserved cooked mushrooms, fresh black pepper, and, if desired, chopped fresh parsley or chives. Serve promptly.
