In a wide, heavy saute pan, bring the wine to a boil with the bay leaves. Put in just enough mussels to cover the bottom of the pan. Cover and cook over medium heat 3 to 5 minutes, or until all the shells are open. Lift out mussels with a slotted spoon and set in a bowl to cool. Cook remaining mussels in the same way, in batches as necessary. Reserve the liquid for another use. Discard shells and place mussels in a bowl with tomatoes, onion, olives, oregano, tomato, orange and lime juices, salt, pepper and olive oil. Refrigerate for a few hours to blend flavors.
Pour vegetable oil to a depth of 1/2-inch in a small skillet. Fry the tortillas on both sides or about 1 minute, or until crisp. Drain on paper towels.
Divide ceviche evenly on tostadas with a dollop of mashed avocado on each.
c.1997, M.S. Milliken & S. Feniger, all rights reserved