In a large bowl, whisk together all of the marinade ingredients. Add the fish to the marinade, and toss it around to coat. Set aside and let it marinate for 20-30 minutes.
In the meantime, make the tostada shells by preheating the oven to 400 degrees. Lay the tortillas flat on a large baking sheet, not overlapping each other. Bake for 7-8 minutes on one side, then flip and bake another 5-6 minutes, until golden brown and crispy. Make sure you watch them, because they can burn easily. Remove from the oven and set aside (and leave your oven on at 400).
While the tostadas are cooking, make the cabbage slaw. In a medium bowl, add the cabbage, mayo, vinegar, salt and pepper. Mix together until evenly combined. Then set it in the fridge until ready to eat.
When the fish are done marinating, place then on a baking sheet in an even layer, and bake at 400 degrees for 12-15 minutes, depending on the thickness of your fish.
While the fish are baking, make the black bean puree. Add the can of black beans, broth, salt, pepper and cumin to a food processor. Process for about 30 seconds, until the beans are pretty smooth and not too chunky. Transfer to a medium saucepan, and heat on the stove over medium-low heat while the fish cook. Stir occasionally, add more broth if it gets too thick.
Now you can assemble the tostadas. Lay a tostada shell on the plate, add 2-3 tbsp of the black bean puree and spread it onto the shell. Break up the fish into smaller pieces, and then layer it on top of the beans, then add about 1/3 cup of the slaw. Top with a couple of tomato pieces, and avocado slices.
Continue with the rest of the tostadas, serve and enjoy!