Beef Shepherds Pie

Name: Beef Shepherd's Pie
Ingredients: 2 tablespoons extra-virgin olive oil; 3 tablespoons unsalted butter; 1 medium onion, diced; 3 medium carrots, peeled and thinly sliced; 1 rib celery, thinly sliced; 1/2 pound cremini mushrooms, quartered; 2 cloves garlic, minced; Kosher salt and freshly ground black pepper; 3/4 teaspoon dried oregano; 3 tablespoons tomato paste; 1 pound ground beef; 1/2 cup canned low sodium beef broth; 1/2 teaspoon Worcestershire sauce; 4 cups Mashed Potatoes, recipe follows; 1/4 cup grated Parmesan, optional; 3/4 cup whole milk; 6 tablespoons unsalted butter; 1 1/2 pounds russet potatoes, peeled quartered; 2 tablespoons chopped fresh flat-leaf parsley