Combine all ingredients for sauce in medium bowl. Stir well, to fully blend ingredients into creamy sauce. Cover; refrigerate for 30 minutes.
Lightly butter kaiser roll halves. "Toast" them lightly by placing them cut side down, in a dry skillet on medium heat. Cook until golden brown and crispy. Remove from heat; set aside.
Generously spread sauce on top/bottom halves of the toasted sides of rolls or buns. Place cooked chicken breast (or 1" thick slice of rotisserie chicken breast, if using) onto bottom half of bun. Add red onion slices, tomato slice, and a large lettuce leaf. Place top half of toasted bun onto sandwich (make sure it has sauce on it!). Repeat process for other sandwich. Serve, and enjoy.
TIP: If using previously cooked chicken breasts or rotisserie chicken, the time it takes to make this sandwich will be considerably shortened.
If using a LARGE, thick chicken breast, simply slice it in half horizontally and use the two halves (a total of one whole breast) for two sandwiches.
Mediterranean Chicken Sandwich https://www.thegratefulgirlcooks.com/mediterranean-chicken-sandwich/