Place the sliced onions into a bowl and squeeze fresh lime juice over them. Make sure all of the onions are covered with the lime juice. Let them sit for about 20 minutes.
Once the onions have been quick pickled, place them into another bowl along with the avocado, cucumber, scallions, and corn. Season lightly with salt and pepper, drizzle about half of a teaspoon of the oil from the mackerel can over everything, and gently toss to coat.
Transfer the salad to two serving plates (or one, if you're eating this as a meal yourself).
With a fork, carefully break off big pieces of the mackerel fillet and place them onto the salads.
Add a few more cracks of black pepper, and serve.