Rinse and pat the chicken breast dry. Using a sharp knife and being careful, slice the chicken breast lengthwise to yield two or three thin slices of chicken. Drizzle with a little olive oil and season with salt and pepper.
Heat your grill pan or skillet to medium high. Cook chicken for approximately 3-4 minutes per side or until cooked through. Remove from heat and set aside
Begin building your sandwich, on the top slice of the toasted bread, spread the guacamole or smashed avocado. Add the sliced red onions and tomatoes.
On the bottom slice of the toasted bread, add one piece of lettuce, one slice of chicken, and top with one slice of bacon that has been cut in half. Depending on the chicken breast you may need to cut it in half to fit the bread.
You can serve this open and combine the top and the bottom slices of bread at the table.
Slice in half and serve immediately.