Prepare your veg: peel and thickly slice the onion. Trim and peel the carrot and thickly dice. Peel and thickly dice the sweet potato. Peel and grate or crush the garlic. Crumble the stock cube into a heatproof jug. Pour in 500ml boiling water. Stir to dissolve. Set aside.
Heat a large pan for 1 min. Add 1 tbsp oil and the lamb. Cook for 4-5 mins, stirring a few times, till browned all over.
Transfer the lamb to a bowl using a slotted spoon. Add the onion to the pan. Season with salt and pepper. Cook and stir for 3-4 mins till just softened. Add the garlic and cook for 1 min.
Return the lamb to the pan along with any resting juices. Add in the carrot and sweet potato. Add ½ tsp harissa – it’s quite hot, so using a little now means you can add extra at the end if you want.
Pour in the chicken stock. Tip in the pearl barley and stir everything together. Cover and cook over a medium heat for 45 mins till the pearl barley and lamb are tender. If it gets too dry, add a little more water.
While the tagine is cooking, tip the sesame seeds into a dry frying pan. Toast for 2 mins, shaking the pan, till nutty smelling and golden. Take care not to burn the seeds – they catch quickly. Tip into a bowl. Cut the orange into wedges.
Taste the tagine and add more salt, pepper or harissa if you think it needs it. Serve in warm bowls, scattered with the toasted sesame seeds and coriander leaves, with orange wedges for squeezing.