Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a frying pan, add the lamb and fry on both sides until browned. Lift out with two slotted spoons and transfer to the slow cooker pot. Fry the onion, stirring, for 5 minutes or until softened and just turning golden.
Stir in the garlic, then the flour. Add the mint jelly and wine and mix until smooth. Pour in the stock, season and bring to the boil, stirring. Pour the sauce over the lamb, cover with the lid and cook on high for 7–8 hours or until the lamb is almost falling off the bone.
When almost ready to serve, put the couscous and beetroot into a bowl, pour over the boiling water, then add the lemon rind and juice, oil and some seasoning. Cover with a plate and leave to soak for 5 minutes.
Add the herbs to the couscous and fluff up with a fork, then spoon on to plates. Lift the lamb on to a serving plate and carve into rough pieces, discarding the bone. Divide between the plates and serve the sauce separately in a jug to pour over as needed.