Cut the onions, garlic, ginger and chillies into slices. If you want, you can also hack anything. Halve or quarter the mushrooms, it depends on the size.
Heat the oil in a wok. Sweat the onion and mushrooms first, then add the ginger, chilli and garlic. Add the stock and the fish sauce. Place the spinach on top of the vegetables and let them fall over medium heat. Mix everything well. If necessary, season with salt and pepper.