Cut lamb into 1-2 inch chunks, trimming off any excessive fat or gristle you find in the process
Heat 1 tablespoon of oil in a large (at least 6 quart) dutch oven
When oil is hot enough to sizzle, begin searing lamb roast chunks, working in batches if you need to. Stir them around to brown them on all sides, but don’t try to cook them through – you just want to sear them.
Remove seared lamb from pan, and set aside in a bowl or other suitable container
Add second tablespoon of oil
Dice onion, and add to pan, sautéing until transluscent
While onion sautés, mince garlic, peel and dice carrots and parsnips into 3/4 inch chunk, and dice potatoes into 3/4-1 inch chunks
When onions are translucent, add garlic, carrots, and parsnips to pot, and stir, continuing to sauté for another 2-3 minutes
Add potatoes, beer, beef broth, tomato sauce, Worcestershire sauce, salt, and pepper, and stir together
Top with thyme and bay leaves
Cover with lid, and bring pot to a simmer.
Simmer until done through – 30-45 minutes.
You can also opt to pour the soup into a slow cooker and let continue to simmer all afternoon. I definitely find that mutton can be a bit tough, and can benefit from 2-3 hours of simmering, as opposed to simply cooking until it’s done, as you might with lamb or beef.