In a small bowl, mix the onion, olive oil, vinegar, and salt and pepper until well combined.
Let sit for 5 to 10 minutes, or until the onion starts to turn pink.
To make the Pan con Aguacate:
In a medium bowl, mix the avocado, lime juice, and salt and pepper until combined.
Slice the bread rolls in half.
To assemble the sandwiches, spread the avocado mixture over the bottom halves of the rolls, add the pickled red onions (if using), and top with the top halves of the rolls.