1.Preheat oven to 220°C (200°C fan-forced).
2.Using mortar and pestle or blender, grind garlic, salt and peppercorns until smooth. Stir in sauce and half of the oil. Rub beef with paste, then cover and refrigerate.
3.Enclose four onion quarters and half the butter in aluminium foil. Repeat with remaining onion and butter. Place on oven tray and roast 1 hour.
4.Meanwhile, boil, steam or microwave potatoes 5 minutes; drain. Slice potatoes thickly. Toss with remaining oil and paprika; cook on heated oiled grill plate (or grill or barbecue) until browned.
5.Cook beef on heated oiled grill plate (or grill or barbecue). Cover and stand 5 minutes. Serve with buttered onions and paprika potatoes.
For nicely crisped potatoes, heat the oven tray before adding the potatoes.Note