In a bowl, mix together mayonnaise, chopped parsley, oregano, red chilli flakes, olive oil, minced garlic, red wine vinegar, salt, and pepper to create the chimichurri mayo. Set aside.

Place the bacon in a large cast-iron skillet and cook over medium heat until crispy. Once cooked, transfer to paper towels to drain and cool. Once cooled, chop the bacon into bite-sized pieces.

In the remaining bacon fat, sauté the onion rings until golden brown. Remove and set aside.

Add the chicken breasts to the skillet, cooking over medium heat and flipping halfway, until they reach an internal temperature of 165°F (74°C) — this should take roughly 8 minutes. Transfer the chicken to a bowl and shred using two forks.

Add salt, pepper, the chopped bacon, sautéed onions, jalapenos, and the prepared chimichurri mayo to the shredded chicken. Combine until well mixed.

Assemble the sandwich by spreading the chicken mixture on a bread slice. Top with another slice, press gently, cut in half, and serve.