Place the baby potatoes in a large saucepan and cover with salted water. Bring to the boil and simmer over medium heat for 15-20 minutes or until the potatoes are cooked through. Remove from the heat, drain, and set aside.
In the meantime, over high heat, in a large sauté pan, drizzle some oil and add the minced beef. Season well with salt and pepper. Cook for 4-5 minutes until well coloured.
Add the onion, garlic, carrot, celery, courgette, and sauté for a further 2-3 minutes.
Then, stir in the corn flour. Cook for a further minute, stirring to coat the ingredients.
Add the harissa marinade, chilli flakes and pour in the stock and the cooked potatoes. Combine and again bring to the boil. Reduce the heat to low, add a lid and simmer for a further 20 minutes until cooked through and the sauce has reduced to coat the back of the spoon.
In a bowl, combine the salad leaves and drizzle some lemon juice and olive oil. Toss and season well. Serve the savoury harissa beef immediately with salad leaves.