To make the salsa, heat the olive oil in a pot on medium high, and then cook the onions and jalapeños for 10 minutes, stirring occasionally. Add the minced garlic and cook for another minute, then turn off the heat and then stir in the crushed tomatoes, scraping any brown bits that might have formed on the bottom of the pan. Mix well but don’t let it get hot.
Immediately place the tomato mixture into a blender, and puree into smooth. Pour the blended tomato mixture back into the pot, and add the olives, capers, marjoram, thyme, cayenne, bay leaf, cilantro, and lime juice. Bring to a boil and then simmer on low for 20 minutes. Taste and adjust seasonings and add salt. Remove bay leaf before serving.
Meanwhile, to make the tostadas, in a skillet, heat up the oil on medium high for 5 minutes. If you flick in a drop of water and it sizzles and pops, it’s ready. One at a time, add the corn tortillas and heat on each side for about a minute. Remove from oil, sprinkle with salt, and drain on a paper towel.
Season each side of the fish fillets with cumin and salt. Drain the oil from the skillet, leaving behind 1 tablespoon. Heat on medium, then add the fillets and cook on each side for about four minutes, or until the fish flakes. (You may have to do this in batches, depending on the size of you skillet).
Take cooked fish fillets, and place one fillet on a fried-corn tortilla, cover in salsa and garnish with lime and cilantro.