Remove the lamb from the refrigerator about 30 minutes before cooking.
Heat 1 tablespoon of the olive oil in a tagine or large heavy-based Dutch oven over medium-high heat. Remove the lamb from the bag and brown in batches until well coloured on several sides. Transfer each batch to a plate.
Reduce the heat to medium-low and add the remaining tablespoon of olive oil. Add the sliced onions and cinnamon stick and cook for 8–10 minutes, stirring regularly, until the onions are softened.
Pour in the stock and stir well, scraping the bottom of the tagine to release the browned bits.
Return the lamb and any accumulated juices to the tagine, nestling the pieces into the onions and broth.
Bring just to a gentle simmer. Cover, reduce the heat to low and cook very gently for 1 hour 30 minutes.
Add the carrots and potatoes together, nestling them amongst the lamb and into the broth. Add a little additional stock if necessary.
Cover and continue cooking very gently for 40–50 minutes, or until the lamb is very tender and the carrots and potatoes are completely tender but still holding their shape.
Stir 1 teaspoon honey into the broth. Taste before adding any more. The finished tagine should have only a subtle background sweetness.
Taste and adjust the salt and black pepper as required.
The broth should remain loose and spoonable. If excessively watery, simmer uncovered for 5–10 minutes. If it has reduced too far, add a splash of stock or water.
Remove the cinnamon stick. Scatter with fresh coriander and serve hot with couscous.