These sandwiches are a knock-off of a simple sandwich that I ate at restaurants as a kid.  You can add mayo or mustard or whatever you like on your chicken sandwiches; for me, it’s the plain-Jane version with just some good lettuce and tomato that does the trick.
The marinade is also a fabulous salad dressing, so you can make extras and store in a mason jar in the refrigerator — it will last for weeks that way.

2 lbs. boneless, skinless chicken breasts, cut into roughly 4-oz portions (you want the chicken to fit the buns, essentially)

Marinade:
1 tablespoon garlic powder
1 tablespoon dried oregano
1 1/2 tsp. salt
1/2 tsp. black pepper
1/2 tsp. crushed red pepper flakes
2 tsp. honey
6 tablespoons red wine vinegar
6 tablespoons olive oil

Sandwich buns of your choice, preferably lightly toasted or grilled
Lettuce leaves
Sliced tomatoes

Whisk the ingredients for the marinade in a medium bowl until thoroughly combined.  Pour over the chicken and toss to coat.  Cover and refrigerate for a minimum of 2 hours, and up to 8 hours.

Grill the chicken breasts over medium heat for 12-14 minutes, just until cooked through.  Allow the chicken to rest for 5-10 minutes before making the sandwiches.  Pile a few lettuce leaves, several slices of tomato, and a piece of chicken onto a lightly toasted bun.  Add condiments if you wish, but I prefer mine plain.  These sandwiches taste best when you eat them in your backyard with a cold drink and good friends.