Bring water, dried porcini mushrooms, and peppercorns to a boil in a medium pot. Reduce heat, and simmer gently for 1 hour. Strain stock (you should have 7 to 8 cups); discard solids. Return to pot; cover to keep warm.
Melt 3 tablespoons butter in a large saucepan over medium-high heat. Cook mixed mushrooms until tender and slightly browned all over, about 3 minutes; transfer to a plate.
Melt 3 tablespoons butter in saucepan over medium heat. Cook onion until tender and translucent, about 5 minutes. Season with salt. Stir in rice, and cook until coated, 1 to 2 minutes. Add wine, and cook until almost completely absorbed, 3 to 4 minutes.
Ladle 1 cup reserved stock over rice, and cook, stirring constantly with a wooden spoon, until almost completely absorbed. Continue adding stock, 1 cup at a time, stirring constantly and waiting for each addition to be almost completely absorbed before adding the next, until grains are al dente but not crunchy, about 30 minutes. (You'll probably only use 6 to 7 cups stock. Add remaining stock to loosen the risotto, if desired.)
Remove from heat, and season with salt and pepper. Stir in remaining 2 tablespoons butter and the grated Parmesan. Top risotto with sauteed mushrooms (reheat if necessary), and garnish with shaved Parmesan.