Preheat the oven to 200°C.
Slice the potatoes into 1 cm rings. Slice the onion into thin wedges. Slice the bell pepper into thin strips.
Lightly grease a large baking tray with olive oil, then cover the base with the potatoes, pepper and onion slices. Drizzle with olive oil and season with salt and pepper.
Put the lamb on top of the veggies, rub the harissa paste all over it then season with salt &pepper.
Drizzle the lemon juice all over the dish then bake for 45-55 minutes, depending on how well cooked you like your lamb. Turn the veggies over and baste them (and the lamb!) with the cooking juices at least once. Add the tomatoes and olives after half an hour of baking.
When the lamb is cooked to your liking, transfer it to a plate to rest and finish off cooking the veggies if they’re not ready yet.
When the veggies are cooked through and look golden, carve the lamb and serve with the vegetables. Scatter the fresh herbs all over.