Boil the cubes of potato for between 8 to 12 minutes until tender. Start poking after 8 minutes to make sure they don't overcook. Drain and put to one side.
Fry the cumin seeds with the knob of butter and oil, until the seeds are just starting to brown.
Add the chopped onion, garlic and ginger and fry until they start to brown.
Add the boiled potatoes, turmeric and garam masala and 1 tablespoon of water to moisten the dish.
Add the spinach and stir for 4 minutes until the spinach has wilted and the potatoes are just starting to crumble. Serve.
You can leave out the fresh ginger if you don't have any. I have made this dish with and without the ginger and it doesn't affect the taste massively.
I also use leftover boiled potatoes when I have them instead of using fresh, a great way to use up those leftovers.
You can also add more garam masala for a more spicy version. This is toned down for the kids.