Tip in the lamb and fry over a high heat for 5 mins, turning over occasionally, until no longer pink and starting to brown. Transfer to a plate with a slotted spoon.
Return the lamb to the pan along with the potato, mushrooms, rosemary and thyme. Cover and cook in the oven for 1¾ hours.
To Make the Dumplings:
Put the flour in a medium bowl and season with salt and pepper.
Add the suet and rub together with your fingers to make rough crumbs. Stir in the rosemary, thyme and parsley, then pour in 2 tablespoons of cold water and mix using a round bladed knife to a soft dough. Cut the dough into 8 even-sized pieces and shape into balls.
Gently place the dumplings on the surface of the casserole and cook in the oven, uncovered, for 20 more mins until they are risen and the lamb is meltingly tender.