Place all pesto ingredients except olive oil into food processor fitted with the steel blade attachment. Pulse until ingredients are combined. With machine on, slowly pour oil in and process, scraping the sides of the bowl when needed. Mix until pasty.

Preheat oven to 400 degrees Fahrenheit. Place portobello mushrooms on a greased cookie sheet and bake for four minutes until just softened. Remove from oven and brush generously with pesto. Sprinkle mozzarella cheese over pesto and bake for four more minutes. Remove from oven.

Just prior to serving, preheat oven to broil and broil portobello pizzas for two minutes, until the cheese starts to turn brown and crispy.