First the easy stuff. Mix the mayo, sour cream and taco seasoning in a small bowl. Set aside.
Spray the salmon pieces with olive oil and coat them in your spice mixture. How much you put on is up to your taste. Enough to cover the top side is probably plenty. Set aside.
You can do this next step ahead or enlist your friends at a dinner party to help. Chop the red onion, shred the cabbage, slice the avocados (Don' do this very far ahead or the avocados will brown.), slice the cherry tomatoes and chop the cilantro.
Now you'e ready for the action. Heat the oil in a large sauté pan. You want the oil one inch deep. Heat the oil until ready for deep-frying. You can tell when it' ready if you sprinkle a few drops of water off your hand into the oil and it sort of spits back. You are going to deep-fry your tortilla shells so you have tostada shells which are fried "Frisbees." You want them to remain hot and crispy, so this is something you will do just before you assemble dinner. Fry them one at a time until brown, turning over halfway through the frying process. They will become stiff. Drain on paper towels. Just stack them with a piece of paper between each one. (I usually toss out the first one. The first is usually a little greasy because the oil isn't fully heated, so I have a few extras on hand in case I need to make more.)
While the tortillas are frying, you can start your salmon. Heat the oil in the pan and place the fillets in the pan. You want to lightly brown them on both sides. How long you cook them depends on how you like your salmon. If you like it medium rare, you will just sear the outsides and leave the inside deep pink. Most folks like the salmon fully cooked, so you'll fry until the salmon flakes easily and is light pink throughout. This will take 5 to 7 minutes in Medium-High heat. If you don't feel confident watching the salmon and the frying tortillas at the same time, do the tortillas first. The salmon will cool rather fast and you'd rather have warm salmon than a hot tortilla.
Now you are ready for assembly. This is not a science. You are just going to stack things so they look pretty to you. Start by coating one side of each tortilla with the mayo mixture; this is your "sauce" of sorts. Put your tortillas sauce side up on your dinner plates. Then put on the cabbage and red onion. Sprinkle on some corn. Then put on the salmon. Drizzle the salmon with lime juice. (Just squeeze the lime over the salmon. No need to juice the lime ahead.) You can leave the salmon as a whole piece or break it into chunks with a fork. The top layer will have the avocado slices, the tomatoes and the remainder of the corn and the cilantro. It's quite colorful.