Portabella Mushroom Pizza Bites

6 mini Portabella mushrooms (or 2 large Portabella mushroom caps)
1/2 cup marinara sauce (if all you have is a tomato, you can try my Xed Pizza Sauce)
1/2 oz. turkey pepperoni (approx. 6 slices)
2 oz. black olives
3 oz. low-fat Mozzarella cheese
1 egg white