Preheat the oven to 375F (190C) and lightly grease, or line with paper liners, a standard sized 12 muffin tin. Set aside.
In a medium skillet, heat a couple tablespoons of water over medium-high heat. Add the onion, garlic and a pinch of salt and sauté for 3-4 minutes, or until starting to soften. Add more water as necessary to prevent sticking. Add the mushrooms and cook until they have released their liquid and most of the liquid has evaporated, about 4-6 minutes. Add the spinach and cook for another minute until wilted. Turn off the heat and set aside.
In a large bowl, combine the chickpea flour, baking powder, salt, and nutritional yeast. Whisk to combine. Add the water and whisk until no lumps remain. Add the mushroom mixture to the bowl and stir to combine. The batter will be very thin and watery. This is ok!
Using a 1/3 cup (80 ml) measuring cup, divide the batter between the prepared muffin cups. Bake in the oven for 28-32 minutes, or until the center bounces back when pressed lightly. You can also stick a toothpick in the center, and if it comes out clean they are done. Don't overbake or they will be dry. Allow to cool in the muffin tin for 10 minutes and then remove to a wire rack. If you have a hard time getting them out, run a knife around the edge of each muffin and they should pop out. These are delicious warm or cold. Enjoy!
Allow to cool completely and then store in an airtight container on the counter for two days, in the fridge for four days or in the freezer for up to one month.