Turkey Avocado Club

Ingredients:
1/3 cup thinly sliced red onion
2 tablespoons apple cider vinegar
8 slices turkey bacon
1 avocado, halved and pitted
1/2 cup nonfat plain Greek yogurt
Kosher salt and freshly ground pepper
8 slices whole-grain bread, lightly toasted
8 small leaves romaine lettuce
1 tomato, cut into 8 slices
12 ounces sliced low-sodium deli turkey breast
1/2 small cucumber, sliced