Pre-heat oven to 375F.
Line baking sheet with foil for easy clean-up. Place fish on foil and coat it with olive oil. Sprinkle some cumin, salt, and pepper on top.
Cook fish until it easily flakes with a fork. Mine took about 15 minutes.
While fish is cooking, make slaw by combining cabbage, cilantro, lime juice, Greek yogurt, garlic, spring onions, and corn.
Make the tostadas by frying corn tortillas in oil for 1-2 minutes per side. Ensure oil is hot prior to putting the tortillas in. Once tortillas are cooked, remove to plate lined with paper towel to soak up oil. You may need to fry them one by one.
Once fish is done, you can start assembling the tostadas by adding the slaw, fish, sour cream (optional) and avocado slices (optional). Add extra salt and pepper if desired.