Heat a large skillet over medium-high heat. Brown the ground turkey and onions until turkey is cooked and onions are tender, about 8-9 minutes. Break up the mixture into small pieces using a potato masher or wooden spoon. Remove from heat and drain any excess liquid.
Pour the taco sauce, corn and black beans into the meat mixture and mix well to combine.
Preheat oven to 450 degrees. Microwave the tortillas for 30 seconds and keep covered as you work with one at a time.
Using a pastry brush, brush both sides of the tortilla with oil. Place some turkey mixture on one half of the tortilla. Sprinkle cheese over top. Gently fold the other half of the tortilla over, gently pressing down. Transfer to an ungreased baking sheet.
Repeat the process with the remaining tortillas. If the tortillas start to break, heat them again in the microwave.
Bake tacos for 20 minutes, until golden brown and crispy. Garnish with fresh cilantro and pickled onions, if desired.