Cook bacon in a skillet over medium heat until crisp and golden brown, about 5-7 minutes. Transfer to a paper towel-lined plate to drain. Once cooled, crumble into bite-sized pieces.
If not already prepared, cook chicken breasts until internal temperature reaches 165°F. Let cool slightly, then dice into 1/2-inch cubes.
Wash and dry romaine lettuce, then chop into bite-sized pieces.
In a large mixing bowl, combine romaine lettuce, halved cherry tomatoes, sliced red onion (if using), and shredded cheddar cheese.
Add the diced chicken and crumbled bacon to the bowl. If using optional ingredients like avocado, cucumber, or hard-boiled eggs, add them now.
Drizzle ranch dressing over the salad and gently toss until all ingredients are lightly coated with dressing.
For best flavor, chill the assembled salad for 10-15 minutes before serving.
Just before serving, sprinkle with croutons (if using) and fresh chives for garnish.
