Using a meat grinder or food processor finely chop cooked meat. Combine lamb and beef with garlic, onion and herbs. Melt butter in a saucepan over medium heat. Add the flour using a whisk to whisk into the bubbling butter and make a white roux. Add broth and simmer for 20 minutes. Place chopped beef mixture into baking dish. Add gravy to beef and adjust seasoning. Decorate top of beef with mashed potatoes. Bake for 45 minutes and serve.