Heat the oil in a medium saute pan over medium heat and saute the red onion and garlic until softened, around 5 minutes. Add the cumin and cook for another minute. Drain the beans into a colander and rinse under cold water. Add to the pan and stir to combine with the other ingredients. Using a potato masher roughly mash the beans to bind them with the onions and garlic, but not too much, so you still retain some of the bean texture.
Place the taco shells on a foil-lined baking sheet and put them in the oven for 2 to 3 minutes to warm.
Meanwhile make the salad. Put all of the chopped and diced salad ingredients into a bowl. In a small bowl, mix together the mayo, lime juice and salt, using a fork. Pour over the salad and toss gently making sure everything is lightly coated in the dressing.
When the taco shells are warm, remove them from the oven.
Divide the bean mix evenly among the 4 tacos, top each one with a couple of slices of pickled jalapeno, a quarter of the sweet corn and a quarter of the salad and garnish with cilantro leaves. Devour.