Heat a large, heavy-based pan with a lid over a high heat and add a good glug of vegetable oil. Then add the beef, in batches to avoid overcrowding the pan, and brown all over. Set aside on a plate.
Add another glug of vegetable oil to the pan and then the onion, garlic and carrots with the dried mixed herbs and sweat over a low heat for about 10 minutes until the onion is soft and translucent.
Return the beef to the pan with any meat juices on the plate, sprinkle over the flour and stir until the beef and vegetables are coated. Gradually add the stock and then the tomato puree and bay leaf, stirring constantly. Bring to a simmer, then reduce the heat, put the lid on the pan and simmer gently for an hour or until the beef is falling apart, stirring occasionally and adding a little water if the stew is too thick. Remove and discard the bay leaf.
Meanwhile, preheat the oven to 200°C/Fan 180°C/ Gas 6. Spread the sweet potato chunks out on a baking tray, drizzle with olive oil and roast for 30 minutes or until cooked through.
Once the beef is done, stir the roasted sweet potato chunks through the stew. Stir the harissa into the remaining mixture, if using, then season to taste. Serve with flatbreads, if you like.
If freezing, portion into containers , cool, seal and label. It takes 8 minutes to reheat in the microwave (stir and re-cover after three minutes) and 40-45 minutes in the oven at 180°C/Fan 160°C/ Gas 5.