1. To prepare topping, place potato and 1 teaspoon salt in a medium saucepan; cover with water. Bring to a boil. Cook 20 minutes or until very tender; drain. Return potato to pan. Add buttermilk, 1/2 teaspoon salt, black pepper, nutmeg, and red pepper to potato; mash with a potato masher until smooth. Set aside.
2. Preheat oven to 375°.
3. To prepare filling, combine lentils, 4 cups water, 1/2 teaspoon salt, and bay leaf in a saucepan; bring to a boil. Reduce heat, and simmer for 25 minutes or until lentils are tender. Drain, and set aside.
4. Combine 1/2 cup vegetable broth and flour in a small bowl, stirring with a whisk until well blended. Set flour mixture aside.
5. Heat olive oil in a large skillet over medium heat. Add onion, carrot, celery, and mushrooms; cook for 7 minutes, stirring occasionally. Stir in remaining 2 cups broth. Add sherry, soy sauce, tomato paste, thyme, and lentils to mushroom mixture. Bring to a simmer over medium-high heat. Stir in flour mixture, and cook until mixture thickens (about 2 minutes), stirring constantly with a whisk.
6. Spoon lentil mixture into a 2-quart casserole; top with potato mixture, spreading evenly. Bake at 375° for 25 minutes or until potatoes are golden. Drizzle truffle oil over potatoes. Garnish with chopped chives, if desired.