1. Preheat oven to 400°F (205°C).
2. Heat a skillet over medium heat. Once hot, add oil and chopped onion and sauté for 3-4 minutes until onions are translucent.
3. Add sliced mushrooms and frozen spinach to a skillet. Cook until mushrooms are soft and spinach is thawed. Season with salt and black pepper to taste.
4. In a mixing bowl, combine chickpea flour, nutritional yeast, baking powder, garlic powder, black salt, salt, and water. Whisk to combine. Brush 12 muffin tin with oil.
5. Divide the mushroom-spinach mixture between muffin cups using a spoon. Now fill the cups with chickpea batter, and use a spoon to gently stir each cup to ensure the butter gets under the veggies. Sprinkle the feta evenly over the tops of the cups.
6. Bake for 30-35 minutes. After 30 minutes insert a toothpick to your muffins, if it comes out clean they are ready.
7. Serve warm. Leftovers can be stored in the refrigerator or freezer.