Guinness Shepherd's Pie

Name: Guinness Shepherd's Pie
Prep time: 15 min
Cook time: 45 min
Total time: 1 h
Cuisine: British
Ingredients: 1 pound ground beef; 1 cup yellow onion (finely chopped); 1 carrot (cut into 1/4 inch rounds); 1/2 Tablespoon tomato paste; 2 cloves garlic (minced); 1/2 Tablespoon cornstarch ((or 1 Tablespoon flour)); 1 1/4 cups Guinness beer ((approximately 8-10 ounces)); 1 teaspoon Worcestershire; 1/4 teaspoon dried thyme; 3/4 cup frozen peas; 1 pound russet potatoes (peeled and cut into 1.5 inch pieces); 3 Tablespoons unsalted butter; 1/2 cup milk; salt; black pepper