Skillet Stroganoff Pie

Name: Skillet Stroganoff Pie
Ingredients: 2 pounds potatoes (unpeeled gold potatoes or russet work great); 1 pound ground beef (I use 80/20); 2 tablespoons salted butter (divided); 1 cup onion (diced); 2 tablespoons garlic (chopped); ½ teaspoon dried thyme; 8 ounces mushrooms (cleaned and sliced); 1 teaspoon salt; ½ teaspoon ground black pepper; 1 tablespoon Dijon mustard; 1 tablespoon Worcestershire sauce; 2 tablespoons all-purpose flour; 1 cup water; ¾ cup frozen peas; ¼ cup sour cream (use full fat so it doesn't curdle); 1 cup milk; fresh parsley (to garnish)