To pickle the onion (make at least an hour before you want to make the salsa, up to 2 weeks before):
Whisk apple cider vinegar, water, salt and sugar together until salt and sugar dissolve (note: mine didn’t really dissolve all the way). Add onion and let sit for an hour at room temperature (mine sat in a glass measuring cup uncovered for slightly over an hour). Drain, cover and chill until ready to use.
To make the salsa:
Combine the onion, tomato, cucumber, corn, avocado and cilantro, if using. Top with salt, cumin, pepper and lime juice. Toss gently to combine.
To make breakfast tacos:
Warm corn tortillas in a pan (no need to grease) over medium heat. Top with a fried egg, salsa, cilantro, and EAT!