Take the chicken breast and slice it lengthwise (similar to butterflying it) so that there are two, equal-sized, thin cutlets. Then cut those in half again, height wise, so that you have 4 semi square/round pieces of chicken.
Using a meat hammer (use a large glass if you don’t have one), gently pound the meat so that it is about ½ inch thin.
In a shallow bowl, whisk the egg. On a separate plate, spread out the panko and season with the salt and pepper.
Take the chicken cutlets and dip them in the egg, then coat in the panko, shake off excess and place on a separate plate.
Heat a large skillet over medium heat for 5 minutes- it may get smoky, so turn on your oven fans/open a window. Add 2 tbsp oil (a thin layer) and add the chicken cutlets. Allow to cook for 2.5-3 minutes per side, until golden brown.
In a small bowl, mix together the plain Greek yogurt, ranch seasoning and lemon juice.
Before assembling the sandwich, toast the buns face down in a pan over medium heat for a minute or so. Keep your eye on it. Smear both side of the bun with the homemade ranch then place the chicken, tomato, red onion and arugula on top.