In a large bowl or baking dish, combine the marinade ingredients.

Using a meat mallet, pound chicken breasts to an even ½ inch thickness.

Add the chicken to the dish with the marinade and turn to coat. Marinate 2 hours in the refrigerator.

While the chicken is marinating, make the sun dried tomato aioli. Place all aioli ingredients in a food processor (or use an immersion blender), and process until combined and smooth. Refrigerate until ready to use.

Remove the chicken from the marinade and sprinkle lightly with salt and pepper.

Heat a grill or grill pan over medium heat. Cook the chicken 3-4 minutes per side until cooked through. Set aside, covered, to rest.

While the chicken is resting, drizzle the onions slices with a little olive oil on both sides and sprinkle with salt and pepper. Add onions to the grill or pan in a single layer and cook about 4-5 minutes until soft and charred, turning once. If you are cooking on an outdoor grill and worried about the onions falling through the grates, slice them thicker (about ¾ inch) and they will stay together better, however they will need a couple of extra minutes to cook.

Optionally, place the ciabbatta rolls, sliced side down on the grill for a couple of minutes until lightly charred and heated.

To assemble the sandwiches, spread some sun dried tomato aioli to taste on each half of the ciabatta rolls, place a piece of chicken on the bottom half. Top with onions and arugula, and add the top half of the bun on top.