Place olive oil, tomato ketchup, soy sauce, sherry vinegar, Worcestershire sauce, dark brown sugar and black pepper in a large mixing bowl. Whisk until smooth and coat the ribs in it. Cover and chill for 2-3 hours to marinade.
When ready to cook the ribs, remove from the fridge and pre-heat the oven to 180°C.
Place the potatoes in a large saucepan and cover with water. Bring to the boil and then simmer for 20 minutes.
Meanwhile, arrange the racks of ribs on a roasting tray and cover with aluminium foil. Roast for 20-25 minutes. Drain the potatoes when tender and allow to steam until they have dried out. Heat the sunflower oil and butter together over a medium-high heat. Saute the potatoes for 3-4 minutes until golden brown and crispy. Add a sprig of rosemary to the pan at the same time and season well.
Remove the ribs from the oven when cooked and allow to rest, loosely covered in foil for 10 minutes. Cut the racks into portions when ready to serve and arrange on a wire rack. Garnish with a few potatoes to the side of the ribs and sprigs of rosemary on top of the ribs.
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