Slice the chicken breasts in half right down the length of it to make 4 thinner chicken breast peices then season both sides with sea salt and fresh ground pepper.
Heat a large skillet to medium high heat, then sear the chicken pieces on both sides for a couple of minutes until they have lightly browned on both sides.
Pour the balsamic vinegar into the skillet and let it come to a boil, then reduce the heat to medium low.
Let the chicken simmer in the vinegar to finish cooking. As the balsamic vinegar simmers, it will get thick and saucy and sweet. Occasionally turn the chicken over to coat and glaze it iwth the balsamic reduction.
As the chicken simmers, add the mayo, Greek yogurt, garlic, lemon juice and sea salt and pepper to the mini food processor or blender and blend until smooth.
Stir the parmesan cheese into the dressing.
Divide up the lettuce, bacon, cucumber and onion between 3 plates and add some croutons. Drizzle with dressing and top with sliced chicken.
If you do not have a mini food processor, crush the garlic into