Combine the vinegar, water and salt in a large glass jar. Close the jar and shake to dissolve the salt. Add the onion and shake once again to mix.
Let the onions marinate at room temperature for at least 1 hour. The onions will become more flavorful as more time passes. Store refrigerated in an airtight container for up to 1 week.
Recipe by Golubka Kitchen at https://golubkakitchen.com/meal-plan-mini-creamy-black-bean-bowls-cauliflower-tacos-raspberry-brownies/