For the pickled onions

• In a small saucepan, stir together the apple cider vinegar, water, sugar, and salt until the sugar is mostly dissolved. Add the onions to the vinegar mixture. Bring the onions to a boil then reduce heat and simmer for 8-10 minutes. Toss the onions around occasionally as they simmer so they cook evenly in the pickling liquid. Set aside.

For the corn

• Cook corn. Place the ears of corn on microwave-safe plate and cover with wax paper. Cook the corn in the microwave for 6-8 minutes, turning the cobs every 2 minutes until the corn is just cooked. Let cool.
• Cut corn. When the corn is cool enough to handle, cut the corn kernels off the cobs (see Notes). Discard the cobs and place the kernels in a medium bowl.
• Mix. Add mayonnaise, cilantro, lime juice, chili powder, and a pinch of salt to the corn. Stir until evenly mixed. Adjust salt to taste. Set aside.

For the black beans

• Cook aromatics. In a large skillet, heat the olive oil over medium-high heat. Add the onions, garlic, oregano, and chili powder. Cook until the onions are softened, stirring occasionally, 3 to 5 minutes.
• Add in. Add the black beans, vegetable broth, and apple cider vinegar.  Mash about 1/4 of the black beans with a fork or back of a spatula. Simmer the mixture over medium heat until slightly thickened and most of the liquid is gone, 5-6 minutes.

For serving

• Warm the tortillas in the microwave or char them on an open flame. Top each tortilla with some black beans, corn, pickled onions, and cotija cheese. Sprinkle additional cilantro and lime juice on top, if desired.