Preheat your oven to 400 degrees.
Prepare a rimmed baking sheet with tinfoil and a rack lightly coated in cooking spray. Set aside.
Wash and wipe the Portobello mushroom caps with a damp paper towel to remove any dirt. Using a spoon, scrape out and remove the gills from the underside of the mushroom caps so there is as much room as possible for the egg!
Brush the outside of the mushroom cap with olive oil and place on your prepared baking sheet.
Place your leaves or green onions, if using, on top of the scraped out side of your mushroom.
Press your prosciutto slices on top and up to the edges of the mushroom cap. You want to create a stronger edge to the mushroom to hold the egg inside and on top of the cap.
Crack your egg and place it on top of the prosciutto.
Sprinkle with pepper and parmesan cheese.
Bake for 15-20 minutes, depending how you cooked you like your eggs and how thick your mushroom cap is.
Serve and enjoy!
Using a spoon, scrape out and remove the gills from the underside of the mushroom caps.Adjust your oven racks to upper middle and lower middle positions. You will be using both racks.Preheat your oven to 475 degrees.Combine oil, salt, and pepper in a large resealable bag.Adjust until blended.Add mushrooms.Adjust mushrooms until coated and allow to sit at room temperature for at least 30 minutes but preferably an hour.Prepare a rimmed baking sheet with parchment paper.Place your mushrooms gill side down on top of your parchment paper.Bake on the lower rack until tender, about 20 minutes.Flip mushrooms gill side up.Carefully stuff each mushroom with filling, as evenly as possible. Both your pan and mushrooms will be hot.Bake on upper rack until stuffing is a nice golden brown color, between 3 and 5 minutes.Serve and enjoy every bite!