Mummy Boomes Traditional Shepherds Pie

Name: Mummy Boome's Traditional Shepherds Pie
Ingredients: 1 tablespoon butter; 1 tablespoon olive oil; 1 onion, diced; 2 carrots, diced; 2 stalks celery, diced; 2 cloves garlic, crushed; Salt and freshly ground black pepper; 1 tablespoon tomato paste; 2 pounds lean ground beef or lamb; 2 tablespoons Worcestershire sauce; 1/2 cup beef stock; 1 1/2 cups garden peas; Cheesy Mashed Potatoes, recipe follows; 4 pounds Yukon gold potatoes, peeled, quartered; 4 tablespoons butter; 1/4 cup heavy cream; 1 cup grated mature white Cheddar