Arrange racks in oven so that one is in the middle of the oven, and one is in the highest position. Preheat oven to 400˚F.
Place the mushroom in a dry skillet over medium-high heat. Cook, stirring frequently, for 3-4 minutes. Add the salt, cumin, and oregano. Continue to cook to allow the mushrooms to release their moisture, stirring frequently. When the mushrooms begin to stick to the pan, after 5-7 minutes, stir in the margarine. Cook an additional minute, until the margarine has completely melted. Season with black pepper. Transfer the mushrooms to a bowl and set aside.
Arrange the tortillas on a baking sheet. Lightly brush both sides with olive oil. Bake on the middle rack for 8 minutes, until the edges begin to curl up. Remove from oven and raise the oven temperature to broil.
Top the tortillas with the mushroom mixture, dividing it evenly among the tortillas. Top each tostada with vegan cheese, then place in the oven on the top rack. They will cook very quickly on the broil setting, so watch them closely. Remove as soon as the cheese is melted and the tortillas are golden brown.
Serve them immediately, topped with sliced avocado and cilantro.