Heat 2 tbsp of the oil in your largest pan, sprinkle in the cumin seeds and let them crackle for a few seconds. Add the chopped onion and the mushrooms and cook for 10 minutes, stirring frequently, until the onion has softened and is starting to turn golden.
Sprinkle in the garam masala, add the garlic, ginger, and chilli and stir to mix, then throw in a bag of spinach. Cover and cook for just a minute or two until the spinach has wilted and there is room in the pan for another. Repeat until all the spinach has wilted, then season with salt. Remove with a slotted spoon to a warm serving dish and keep warm.
Fry the onion slices slowly with the turmeric in the remaining oil until the onion is golden brown. Serve the spinach hot, sprinkled with the onions.