Heat a dutch oven over medium/high heat. Add 1 tablespoon of oil and half the lamb, season with salt and pepper, fry until browned, set aside and repeat with the remaining lamb.
Add onion, garlic and rosemary to the pan and sauté until softened.
Return the lamb to the pan, add tomato paste, stout, stock, Worcestershire, bay leaf and seasoning. Bring to the boil, cover with a lid and simmer for 30 minutes. Preheat oven to 180°C.
Dissolve flour with a little of the sauce from the lamb in a small bowl. Add to the casserole and simmer until thickened.
Add carrots and parsnips, taste the sauce and adjust seasoning if required.
Either leave the lamb in the dutch oven or transfer to an oven safe casserole. Top with sliced potatoes, brush with butter and sprinkle with thyme.
Bake for one hour or until the potatoes are tender and golden.
