Preheat oven to 400? F.

Line a baking sheet with parchment paper.

Gently remove the stems from the mushroom caps, and place the caps on the baking sheet, top side down.

Chop the mushroom stems small.

Heat the oil in a large skillet over medium heat.

Add the mushroom stems, onion, sundried tomatoes, and garlic to the skillet, and saut? until the onions are translucent.

Turn off the heat, and add the spinach, parsley, feta, and black pepper. Stir until well combined.

Carefully fill each mushroom cap with a spoonful of the filling. It can be packed down a bit.

Bake for 20-30 minutes, until the mushrooms are browned and not releasing any more liquid.

Serve immediately.