Begin the potato curry by grinding the ginger and garlic cloves into a paste in a pestle and mortar. Set aside
Heat the vegetable oil in a large saucepan on medium heat. Add the cumin and sauté for ½ a minute, then add the diced potatoes and sauté for 2 minutes
Add salt, turmeric and chilli powders to the pan, and sauté for a further 2 minutes
Add ginger-garlic paste, stir well and sauté the potatoes for a further 2 minutes
Add the fine beans and sauté for 1 minute. Then add the chopped tomatoes and tomato paste, if using, stirring well. Continue to cook until the potatoes are tender
Remove the pan from the flame and leave aside for 1-2 minutes. Add salt to taste. Prior to serving, sprinkle with garam masala and the juice of ½ a lime