Preheat the oven to 350 degrees. In a large skillet, heat the olive oil over medium-high heat.
Add the onions and cook, stirring occasionally, until they start to brown, about 5 minutes. When the onions begin to brown, add the tomatoes and corn kernels and sauté them for about 2 more minutes. (At this point you may want to keep some of the corn and beans separate for picky eaters.)
Add the beans, salsa and cilantro (optional) and stir until heated through.
Heat the taco shells on a baking sheet in the oven for about 5 minutes (or warm the tortillas). (Set the timer because they burn easily.)
Assemble the tacos at the table, starting with the bean mixture, then adding the cheese, lettuce and extra salsa, if desired.