Preheat oven to 375°F.
Line a regular muffin pan with 10 muffin papers or use a nonstick muffin pan.
Heat a skillet over medium heat. Add the avocado oil, onions, and garlic. Cook until onions are soft and translucent.
Add the spinach and cook until wilted. Stir occasionally, set aside, and let cool.
In a large mixing bowl, whisk eggs until smooth and season with salt, pepper, and cayenne if using.
Add in the spinach mixture and stir until combined.
Add the almond flour, baking powder, and 1/2 cup cheese. Stir until combined. Let the batter sit for about 5 minutes, then stir again.
Scoop the batter into the 10 prepared muffin cups using a small scooper or a tablespoon.
Sprinkle a little extra cayenne, and top with the remaining shredded cheese.
Bake for 15-20 minutes or until a toothpick comes out clean when inserted into the center of the biscuit. The biscuits should be golden brown.
Serve immediately.