Cut steak into cubes. Heat oil and fry mustard seeds till they burst. Add fenugreek, garlic, ginger and onion. Fry until onions are golden. Add turmeric and fry 1 minute. Add coriander, cumin and chilli. Stir well. Add salt and vinegar, and fry until liquid evaporates. Sprinkle in the garam masala and mix well. Add meat and turn till coated. Add extra vinegar if spices start to stick. Reduce heat, cover and simmer for 1.5-2 hours. Add cubed potatoes and cover. Cook for further 20-30 minutes or until potatoes are cooked. Garnish with fresh coriander.