Slice the cabbage thinly and dice the potatoes. Heat the oil in a large frying pan and cook the spices until the mustard seeds stop popping. Add the garlic, onion and potato and cook for a further 3 minutes. Now add the cabbage and seasoning, cover the pan and cook for another 15 minutes. When the potato is fully cooked, add the lemon juice. Stir briskly and serve.