Add the ACV, cucumbers, dill, parsley, green onion, lemon juice, 1/3 of salt, pepper, garlic powder and tarragon to a bowl and toss together. Set aside to marinate.
Pound the chicken breasts to be as thin as possible (about 1/4”). Slice into “bread” sizes (if you had two whole chicken breasts, pound and then slice each in half, giving you 4 pieces).
Season chicken with the rest of the salt and pepper and grill for about 3 minutes, flip and grill for 2 more minutes.
Assemble the chicken sandwiches with grilled chicken as bread and marinated cucumber sliced inside. Drizzle any extra cucumber marinade juice over the top of the sandwich. There may be extra cucumber salad that doesn’t fit in the sandwich that you can eat on the side. Enjoy while the meat is hot or chilled.
– AP version: add sliced tomatoes and lettuce.
– Bare foods sweet mustard dressing also would be good as a dressing/marinade.