Combine all ingredients for Mediterranean sauce in a medium bowl. Stir very well, to fully blend ingredients into a creamy sauce. Cover and refrigerate for 30 minutes. Note: It is not vital to refrigerate it for 30 minutes, but I do recommend it for best flavor! (If you choose to make sauce up early in the day, cover bowl and refrigerate until ready to use).
Lightly butter the 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.
To assemble sandwich, generously spread Mediterranean sauce on top and bottom halves of the toasted sides of rolls or hamburger buns. Place the cooked chicken breast (or 1" thick slice of rotisserie chicken breast, if using) onto the bottom half of the bun. Add thin red onion slices, a tomato slice, and a large leaf of lettuce. Place the top half of the toasted bun onto the sandwich (make sure it has sauce on it, too!). 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/