In a large pan (with a lid for later), heat the oil on a medium heat. Add the cumin seeds and mustard seeds and fry briefly until they just begin to sizzle and pop, being careful not to burn them.
Next, add the chopped onion along with a pinch of salt and cook for 5-6 minutes or until starting to turn soft and slightly golden.
Once the onions have softened, stir in the chopped garlic, ginger, and chilli and cook for another minute.
Add the chopped potatoes and stir well, followed by the ground coriander, fenugreek, and turmeric. Mix well and add the water, then cover with a lid and reduce the heat to low.
Simmer with the lid on for 10-12 minutes or until the potatoes are beginning to soften, checking on them every now and then. The cooking time will depend on the size of your potato pieces, so just keep an eye on them. They should be tender and easy to pierce with a knife, but not mushy and falling apart.
Remove the lid and begin adding the chopped spinach to the pan. Continue adding this and cooking for a few minutes until it has all wilted down. While I like the final mix to be fairly dry, I sometimes add an extra splash of water at this stage.
Stir in the garam masala and lemon juice and cook for another minute. Check the potatoes are cooked through, adjust the seasoning, and serve with fresh coriander. Enjoy as part of a larger meal, or as a main along with sides such as homemade pilau rice.