Heat the oven to 375°F.

Heat 2 tablespoons of avocado oil over medium heat in a large cast iron skillet and sauté the onions until soft and translucent.

Add the garlic and mushrooms and cook until the mushroom moisture evaporates.

Season the filling with salt and pepper to taste and spoon it on a plate to cool to room temperature.

For the batter, beat the eggs in a large bowl with coconut milk, coconut flour, salt, and pepper until well-mixed. Stir in the sautéed mushrooms and spinach and mix to combine.

Brush the remainder of the avocado oil onto the muffin tin and line each cup with prosciutto, taking care to cover the bottom and sides completely. 

Spoon in the frittata batter, filling each cup until 3/4 full. Top each muffin with some halved cherry tomatoes.

Bake the muffins in the oven for about 20 minutes, flipping the tray at the halfway point (i.e. rotating the tray 180 degrees).

Cool the muffins in the pan for a couple minutes before transferring them to a wire rack.