Shepherds Pie

Name: Shepherd's Pie
Ingredients: 1 1/2 pounds russet potatoes, peeled and quartered; Kosher salt and freshly ground black pepper; 3/4 cup milk; 7 tablespoons unsalted butter; 1/4 cup extra-virgin olive oil; 1 onion, chopped; 3 carrots, sliced into thin rounds; 1 rib celery, thinly sliced; 8 ounces white button mushrooms, quartered; 2 cloves garlic, finely chopped; 3/4 teaspoon dried oregano; 3 tablespoons tomato paste; 1 pound ground beef; 1/2 cup low-sodium beef broth; 1 teaspoon Worcestershire sauce; 2 tablespoons chopped fresh parsley