Grilled Chicken Sandwich

I think I don't make sandwiches enough.  I don't mean the peanut butter type, or the grilled cheese type, I mean yummy, good-for-you, chock full of stuff sandwiches.  After eating soups and heavy meats for a few days (snow-induced eating), we were all ready for something lighter.  After searching through my fridge, this is what I came up with.

Grilled Chicken Sandwiches

Ingredients:
2 chicken breasts
1/2 cup apple cider vinegar
1/2 cup olive oil
1 tsp thyme
1 tsp garlic
1/4 t salt
1/4 t ground black pepper
1 cup mixed greens OR your favorite sandwich lettuce
1 large tomato, sliced
1 avocado, halved and pitted, then sliced into 1/8"-thick slices
Mayonnaise (I used the Mayo with Olive Oil, which tastes delicious and has less fat than original)
Dijon-style mustard
4 sandwich buns (I used whole wheat with poppy seeds)

Butterfly the chicken breasts by cutting them horizontally through the thickness, to create 4 evenly-sized pieces of chicken.  Place in a large ziplock bag and add vinegar, oil, thyme, garlic, salt, and pepper.  Seal tightly and shake to combine.  Let marinate in fridge for at least 30 minutes or up to 2 hours.
Grill the chicken on a grill or under a broiler for 4-6 minutes per side, or until the chicken reaches 165-degrees F.  Set aside for 5 minutes.  If desired, slice into thin slices, or leave whole.
Spread thin layers of mayonnaise and mustard on top and bottom of buns.  Divide the salad greens amongst the buns, and then place chicken on top.  Add tomato and avocado, then the top of the bun. 
Serves 4.