Bring a pot of water to a boil over medium-high heat, then add the chicken breasts. Reduce heat to low and simmer for about 12-15 minutes until fully cooked. Remove the chicken and let it cool slightly, then shred it using two forks.
In a large mixing bowl, combine the shredded chicken, mayonnaise, finely chopped celery, diced red bell pepper, lemon juice, salt, and black pepper. Mix thoroughly until the ingredients are evenly coated and the filling is creamy and well blended.
Toast the bread slices lightly until golden brown using a toaster or oven. Once toasted, arrange the slices on a serving plate.
Spread a generous layer of the chicken salad mixture onto the bottom slice of bread. Top with optional garnishes like lettuce, tomato slices, or fresh herbs for added color and flavor.
Place the second slice of bread on top to complete the sandwich. Press gently to settle the filling, then cut diagonally or in half to serve. Present on a plate with additional garnishes if desired.
Serve immediately with your favorite sides or enjoy as is for a quick, satisfying lunch.