Preheat a grill pan or skillet over medium-high heat.
Season both sides of the chicken breasts with salt, black pepper, and garlic powder.
Add olive oil to the pan and cook the chicken breasts for 5 minutes on each side or until fully cooked and internal temperature reaches 165°F (74°C). Remove from heat and let rest for 5 minutes.
While the chicken rests, toast the bread slices until golden brown.
Slice each chicken breast into 1/4-inch thick pieces.
Lay one slice of toasted bread on a clean surface. Spread 2 teaspoons of mayonnaise evenly on the bread.
Place 2 leaves of romaine lettuce on the bread, then layer half of the sliced chicken on top.
Add 2 slices of tomato and 2 slices of turkey bacon over the chicken.
Top with another slice of toasted bread and spread 2 teaspoons of mayonnaise on the top side.
Repeat the layering with the remaining lettuce, chicken, tomato, and turkey bacon.
Finish by placing the last slice of toasted bread on top.
Secure the sandwich with toothpicks in each corner and cut diagonally into quarters.
Serve immediately.