Preheat oven to 350 degrees and line a baking sheet with parchment paper.
Remove stems from mushrooms and dice into small pieces.
In a medium skillet, add oil and heat on medium-low. When shimmering, not burning, saute onion, mushroom stems and sun-dried tomatoes until tender, about 5-7 minutes.
Add minced garlic and continue to saute an additional minute.
Add spinach and saute just until wilted. Remove from heat and add the feta cheese reserving a 2 tablespoons for topping, breadcrumbs and 3 tablespoons of Parmesan, stir to combine.
Stuff mushrooms with mixture, place on prepared baking sheet, sprinkle with remaining tablespoon Parmesan, top with remaining feta and heat until cheese is melted, about 15 minutes. *Mushrooms are very hot coming out of the oven so be sure to cool a few minutes before serving. This appetizer is yummy warm or cold.
Note: Sometimes the mushrooms may release liquid, so when removing from the cookie sheet lightly dap with a paper towel before serving.
