To make the dressing. Combine all ingredients in a glass jar and whisk until smooth. Taste and adjust the salt and pepper.In a large bowl, toss the chicken with 1/2 of the dressing. Let sit 10 minutes. Set your grill, grill pan, or skillet to medium-high heat. Grill the chicken until lightly charred and cooked through, turning halfway through cooking, about 10 to 12 minutes. To make the tahini ranch, combine everything in a bowl, whisking in 1/4 to 1/2 cup water to thin the dressing. Taste and season with salt and pepper.Rub each half of the bread with olive oil. Grill the bread, cut side down, for 3-5 minutes or until light grill marks appear. Working with the bottom piece of bread, spread over some of the tahini ranch, then layer on the avocado, sun-dried tomatoes, and pepperoncini. Add the chicken, then drizzle the remaining dressing over the chicken. Add the arugula and cheese. Drizzle with the tahini ranch. Add the top half of the ciabatta and gently push down on the sandwich. Cut into 4-6 sandwiches. Enjoy with the remaining tahini ranch on the side.