Place rump on a plate and brush with 1 tbsp olive oil.
Cut potatoes into chunky chip shapes and mix in a bowl with grated lemon zest, rosemary, 1 tbsp of olive oil, salt and pepper. Spread on a large baking tray and roast for 30 minutes at 200C fan forced until deeply golden and crispy.
Heat remaining oil in a large frying pan over high heat, add butter, onion and thyme leaves and cook, stirring often for 5-8 minutes until onion has softened and is starting to turn golden. Add mushrooms and allow to brown, do not stir too much. Season. Cook for 5 minutes, stirring only occasionally. Once mushrooms have good colour, add mustard, worcestershire and wine, allow to bubble up and reduce. Add stock and stir, scraping the bottom. Remove from heat. Allow to cool slightly, stir in yoghurt.
Heat a lightly oiled bbq or chargrill pan over high heat and cook steak for 3 minutes each side, until medium rare, or to your liking. Serve steak, thickly sliced, with chips, sauce, broccolini and beans.
Tips
If your steak is less than 2cm thick 2-3 minutes per side should be fine, but if 2-3cm thick it might need 3-4 minutes per side medium rare.
