Dice the potato into bite-sized chunks. Cut the cauliflower into similar-sized pieces. Boil/steam/microwave these until the potato is beginning to soften - if you overcook the potato, it will fall apart later on.
The sauce is best cooked over moderate hear in a deep frypan or heavy-based wok. Finely chop the onion, and fry it in enough oil to generously cover the bottom of the pan (about 50mL). When the onion begins to turn clear, add all the spices and fry until the seeds begin to crackle.
Add the potato, cauliflower, spinach, and coconut milk at this stage. Stir to mix all the ingredients thoroughly, cover, and simmer until the potatoes are fully cooked. I added a teaspoon each of salt and brown sugar at this stage, and tasted it to see if more chilli powder was needed.
Serve in a deep bowl on brown rice. You should be able to feed four hungry people with these quantities, or more if you're using enormous potatoes.
Modifications:
To make your own garlic/ginger and onion pastes, just use a food processor.
Peel garlic and ginger, coarsely chop them, and process until smooth. Use slightly more garlic than ginger. 2tsp is enough for this recipie.
Chop onions, process them until smooth. Use a couple of tablespoons for this recipie.
These will both keep in the fridge for 4-6 weeks, in a sealed container. You can use them for all sorts of stir-fry and indian dishes.