Parboil the potatoes for 10 minutes then allow to cool and cut into cubes.
Fry the cumin seeds in a saucepan in the oil and butter until just starting to brown.
Add the onion and fry until it too starts to brown.
Add the potato, garlic, turmeric, ginger and garam masala and fry until the potato is warmed through. If you are using fresh spinach, add a few tbsp water. If using frozen, just lob it in.
Cook until the spinach has defrosted/collapses over the potato and season to taste with salt & pepper.