Risotto with Leeks, Shiitake Mushrooms, and Truffles

INGREDIENTS

• 2 large leeks (white and pale green parts only), halved, thinly sliced crosswise (about 2 cups)
• Chopped fresh parsley
• 1 pound shiitake mushrooms, stemmed, cut into 1/4- to 1/3-inch-thick slices
• 4 tablespoons (1/2 stick) butter, divided
• 1 teaspoon minced fresh thyme leaves
• 5 cups (or more) hot vegetable broth
• 1 tablespoon white truffle oil
• 1/4 cup (1/2 stick) butter, melted
• 1/2 cup grated Parmesan cheese
• 1/2 cup dry white wine
• 1 large onion, chopped
• 3/4 cup whipping cream
• 2 teaspoons shaved or chopped black truffle (optional)
• 1 1/2 cups arborio rice or medium-grain white rice
• 1 large onion, halved, thinly sliced lengthwise