My Mothers Shepherds Pie

Name: My Mother’s Shepherd’s Pie
Ingredients: 3 pounds russet potatoes; Kosher salt; 3/4 cup milk; 1/2 cup sour cream; 6 tablespoons unsalted butter, melted; 2 large egg yolks; Freshly ground pepper; 2 tablespoons unsalted butter; 2 tablespoons extra-virgin olive oil; 1 cup pearl onions (fresh or frozen), peeled; 2 tablespoons canola oil; 1/2 yellow onion, diced; 2 sprigs fresh rosemary, minced; 2 pounds ground lamb; Kosher salt and freshly ground pepper; 2 tablespoons tomato paste; 1 1/2 tablespoons all-purpose flour; 1 tablespoon sherry vinegar; 1 cup chicken stock; 1 cup frozen peas and carrots; 1/2 cup fresh parsley, chopped; 3 tablespoons unsalted butter, melted; 1/4 cup grated Parmesan cheese; Chopped fresh parsley, for garnish