Preheat a grill pan or skillet over medium heat.
Season the chicken breasts evenly with salt, black pepper, and garlic powder on both sides.
Add olive oil to the pan and cook the chicken breasts for 6-7 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 cooks, cook the turkey bacon in a separate skillet over medium heat until crispy, about 4-5 minutes. Drain on paper towels.
Toast the bread slices until golden brown.
Spread 1 tablespoon of mayonnaise on one side of each toasted bread slice.
Slice the cooked chicken breasts into thin strips.
To assemble each sandwich, place one slice of bread, mayonnaise side up, on a clean surface. Layer with 2 leaves of romaine lettuce, 2 slices of tomato, half of the sliced chicken, and 2 slices of turkey bacon.
Top with a second slice of bread, mayonnaise side down. Press gently and cut each sandwich diagonally into two triangles.
Serve immediately.