Simmer chicken in a pot of water over medium heat to an internal temperature of 160°F. Remove from water. Dice chicken into small bitesized pieces and refrigerate to cool fully.
Once cooled, thoroughly mix the chicken, scallions, celery, carrots, tomato, blue cheese, buffalo sauce, and mayo, in a bowl. Season with salt and pepper to taste. Add more mayo or buffalo sauce to your preference.
Lightly toast the bread.
Layer the bread with spring mix and chicken salad. Garnish with blue cheese.
Notes
Pro tip: Cut the chicken breasts into similar sized chunks to cook faster. (If the chicken is diced or cut too small prior to simmering in water it has the potential to dry out or overcook).