Preheat oven to 200⁰C. Heat half the oil in a 6 litre heat proof casserole dish on top of the stove. Add the lamb in batches and cook until browned. Remove and set aside.
Add half the remaining oil and to the pan with the onion and garlic and cook over a medium heat for 5 minutes, or until the onion is soft and golden. Add the spices and cook for 1 minute or until fragrant. Return the lamb to the pan, with the tomato paste and 3 cups (750ml)water and stir to combine. Cover and transfer to the oven, cook for 1 hour .Add the sweet potato and dates and cook for 30-40 minutes longer or until the lamb is tender and the sweet potato is soft. Fold though half the chopped parsley.
Put the parsnips, zucchini and red capsicum onto a baking tray, drizzle with the remaining oil and cook for 15-20 minutes or until soft.
Just before serving, Put the couscous, lemon zest into a bowl, cover with 2 cups (500ml) boiling water, set aside until all of the water has been absorbed. Flake the grains with a fork to separate then fold the roasted vegetables. Serve the lamb tagine with the roasted vegetable couscous., sprinkle with the chopped parsley.