Preheat the oven to 170C/325F/Gas 3.
Sprinkle the flour onto a plate and dredge the chopped lamb pieces in the flour until coated. Season, to taste, with salt and freshly ground black pepper.
Heat the oil in a large lidded casserole and fry the lamb, turning frequently, for 3-4 minutes, or until browned all over. (You may need to do this in batches.) Remove each batch of lamb from the casserole and set aside.
Add the shallots to the casserole the lamb was cooked in and fry for 2-3 minutes, then add the rosemary and carrots and continue to cook for a further 1-2 minutes.
Return the browned lamb to the pan, pour in the chicken stock and bring the mixture to a simmer.
Cover the casserole with a lid and transfer to the oven to cook for 1¼-1½ hours, or until the lamb is tender and the sauce has thickened slightly.
Meanwhile, for the rosemary dumplings, sift the flour, baking powder and salt into a bowl. Stir in the suet and rosemary and add enough water until the mixture comes together as a dough.
Using floured hands, roll spoonfuls of the dough into small balls.
Remove the casserole from the oven and season, to taste, with salt and freshly ground black pepper. Place the dumplings into the casserole, cover with a lid and return to the oven for a further 20-25 minutes, or until the dumplings are cooked through. Remove from the oven and set aside.
For the roasted onions and parsnips, increase the oven to 200C/390F/Gas 6.
Mix the onions and parsnips with the oil in a baking tray. Season, to taste, with salt and freshly ground black pepper and roast in the oven for 25-30 minutes, or until golden-brown and tender.
To serve, reheat the lamb rump navarin, if necessary. Divide the roast onions and parsnips among 4 serving plates. Spoon the lamb navarin alongside and top with a few dumplings.