Cook the potatoes in boiling water with a little salt for approx 7-8 minutes or until only just cooked. Remove from the heat, drain and set aside.
In a non-stick fry pan heat the ghee or oil. Add the mustard seeds and the cumin. When the mustard seeds start to 'crackle and pop' add the sliced onions. Stir well and cook until the onions are translucent and starting to brown. Add the garam masala, turmeric. salt and pepper. Mix well and cook for a further 2 minutes.
Add the ginger, garlic and chilli. Cook for a further minute then add the potatoes and spinach. Mix well and keep stirring until the spinach has defrosted and is starting to coat the potatoes.
Add a splash of water and turn up the heat. Keep stirring until the mixture has browned well. Remove and serve or chill for reheating later in the microwave.