Simmer chicken in a pot of water over medium heat to an internal temperature of 160°F. Remove from water. Dice chicken into small bite-sized pieces and refrigerate to cool fully.
Once cooled, thoroughly mix the chicken, scallions, onion, carrots, and mayo, in a bowl. Season with salt and pepper to taste.
Lightly toast the croissant.
In a bowl, mix together all the ingredients for the cucumber tomato salad. Season with salt and pepper to taste.
Layer the bread with spring mix and chicken salad. Serve with cucumber tomato salad and pickles.
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).