In a small bowl mix vinegar, water, and salt. Add sliced red onions and let sit for at least 10 minutes.
Toss corn with olive oil and salt. Roast in a hot skillet or oven until charred and golden. Let cool completely.
In a mixing bowl combine lime juice and minced garlic. Let sit for 2 to 3 minutes.
Add avocados and gently mash with a fork, keeping small chunks.
Stir in cilantro, serrano, salt, and half of the roasted corn.
Transfer to a serving bowl. Top with remaining corn, drained pickled onions, cherry tomatoes, feta, and flaky sea salt.
Serve immediately with tortilla chips or tacos.