Set your oven to 325°F (160°C) to preheat.
Heat olive oil in a large casserole over medium heat, add the bacon and cook until it’s slightly crispy.
Toss in the diced lamb to brown it well on all sides, then pour any meat juices into a separate bowl.
Pour a bit of Guinness or beef broth into the pan and scrape up all the browned bits from the bottom to capture the flavor, transferring this liquid to the bowl with the lamb.
Add a little more oil if needed and gently cook the leeks for about 5 minutes until softened.
Mix in carrots, potatoes, thyme, salt, and pepper, cooking everything together briefly for around one minute.
Pour the Worcestershire sauce, remaining Guinness, and beef broth into the pan, then bring everything to a gentle simmer.
Stir in the tomato paste if you’re using it, then return the lamb, bacon, and add the bay leaf.
Seal the dish with a lid and place it in the oven to cook for 90 minutes, until the lamb is tender.
Remove the stew from the oven, taste and adjust seasoning to your preference.
To thicken the gravy, stir in the cornstarch slurry and warm the stew until it thickens nicely.
Sprinkle the chopped fresh parsley on top and serve your stew with crusty bread for a comforting meal.
