I found this recipe on the Vegetarian Times website and tweeked it a bit. I'm typically a bread slore, but decided to give this recipe a go. Pardon any grammatical errors...food coma.
4-5 portobello mushrooms
1/2 red bell pepper
mozzarella or mozzarella substitute
Clean the mushrooms for obvious reasons. Then cut the stems off the caps, chop the stems and place in a bowl. Chop scallions, red bell pepper, and kalamata olives - mix in same bowl with the mushroom stems. Preheat oven at 400 degrees. Place mushrooms caps gill side up on a baking sheet and spoon pesto on said mushroom caps. Arrange fresh spinach, mixture, mozzarella, mixture, then mozzarella (Yes, in that order. Just do it!). Sprinkle oregano on top and bake for about 15 minutes or until the mozzarella is melted.
Here is a visual aid:
SAFETY TIP: Don't burn yourself.