Add the onion to a small bowl and pour vinegar over it and toss. Let it marinate for 5 minutes or so, then drain vinegar.

In a large nonstick skillet, place a tortilla over medium heat. Working on one half of the tortilla, start and sprinkle half of the cheese, then half of the pork, and top with the onions. Fold the tortilla in half, flatten gently with a spatula and cook until cheese starts to melt. Flip over and cook until the second side is golden. Repeat with remaining tortilla.

Serve warm with a drizzle of lime juice if prefered.