Shepherds Pie

Name: Shepherd’s Pie
Ingredients: 5 tablespoons unsalted butter, divided; 2 1/2 pounds russet potatoes (4 to 5 medium); 1 tablespoon plus 1 1/2 teaspoons kosher salt, divided, plus more as needed; 3/4 cup whole or 2% milk; 1 large egg yolk; 3 medium carrots; 1 medium yellow onion; 2 cloves garlic; 1 1/2 pounds ground lamb, lean ground beef, or a combination; 1 teaspoon dried thyme; 3/4 teaspoon freshly ground black pepper; 1/2 teaspoon dried rosemary; 2 tablespoons all-purpose flour; 1 tablespoon tomato paste; 1 cup beef broth; 2 tablespoons Worcestershire sauce; 1 cup frozen peas (do not thaw); 1/2 cup frozen corn kernels (do not thaw); Chopped fresh parsley leaves, for serving (optional)