Mushroom and Leek Gratin

INGREDIENTS

• 4 tablespoons extra-virgin olive oil
• 2 pounds large portobello mushrooms (6 to 8), stemmed and sliced 1/2 inch thick
• 2 leeks (white and light-green parts only), cut into 1/4-inch-thick half-moons and rinsed
• Coarse salt and ground pepper
• 1/4 cup dry white wine, such as Chardonnay
• 3/4 cup heavy cream
• 2 tablespoons fresh lemon juice
• 1 tablespoon fresh thyme leaves
• 1 ounce Parmesan, finely grated (1/4 cup)