Preheat a barbecue with all burners on to 180ºC. Lightly spray beef with oil and sprinkle with Cajun seasoning to coat. Season with salt and pepper. Place beef in centre of barbecue (could also cook in roasting dish) and turn burners directly under it off. Leave remaining burners on to conduct and circulate heat around beef. Close lid and cook beef for 50 minutes for rare, 65 minutes for medium and 75 minutes for well done. Remove beef, cover loosely with foil and rest for 15-20 minutes before slicing.
Meanwhile, lightly spray sweet potato with oil, season and cook on the barbecue for 10-15 minutes or until tender.
Combine beans, tomato and avocado in a bowl. Season. Combine sour cream, lemon zest and juice in a screw top jar. Season. Drizzle half the dressing over salad.
Serve sliced beef with sweet potatoes, salad, remaining dressing, lemon wedges and parsley.
To cook roast in oven - preheat oven to 180°C (160° fan-forced). Place beef in oven and cook for 50-65 minutes, basting occasionally with pan juices until cooked to your liking. Remove from pan, cover loosely with foil and set aside to rest for 20 minutes.
For ease and accuracy use a meat thermometer. Check temperature when estimated cooking time is up, 60°C for rare, 65°C - 70°C for medium and 75°C for well done.
You could also use a beef topside, oyster blade or round roast for this recipe – cooking time will be the same.
Leftover sliced beef would make a delicious sandwich, roll or wrap the next day – just add some mustard, lettuce and tomato.