Slow Cooker Shepherds Pie

Name: Slow Cooker Shepherd's Pie
Ingredients: 1½ pounds beef chuck, trimmed and cut into 2-inch pieces; 2 large carrots, chopped; 1 large onion, chopped; 1 cup low-sodium beef broth; 1 cup dark beer (such as Guinness); 3 tablespoons Worcestershire sauce; 2 tablespoons tomato paste; 2 tablespoons all-purpose flour; Kosher salt and black pepper; 2 large russet potatoes, peeled; 2 cups frozen peas; ½ cup milk; 2 tablespoons unsalted butter