- Lightly coat the bottom of a large skillet with the extra virgin olive oil. Set the burner to medium-high heat.
- Add the red pepper, red onion and chopped mushroom stems to the skillet. Add a pinch of salt and pepper. Sauté until the veggies soften, stirring occasionally, about 5-7 minutes.
- Add the baby spinach. Continue to sauté the veggies until the spinach wilts and the onions are translucent. Remove them from the heat and set aside.
4. Turn the burner down to medium heat. If the skillet needs more oil, add it at this time. Place the portabella mushroom caps in the skillet, inside of the cap facing down. Let the caps heat up and soften for about 3 minutes. Flip them over, add a little salt to the inside of the caps and cover the skillet with a lid. Let the caps steam to soften for another 3 minutes.
5. Turn on the broiler. Place the mushroom caps on a baking tray. Divide the filling between the four caps and top each one with a slice of provolone. Put the caps under the broiler until the cheese begins to brown, about 3-5 minutes.
NOTE: Steps 4 and 5 can be accomplished using a grill!