Heat a pot of water over high heat until boiling.
Cook the chicken: When the water is hot, add the chicken and cook at a simmer for until the chicken has reached an internal temperature of 160°F (71℃). Remove from water and then diced into small pieces and transfer to the fridge to cool.
Prep the ingredients per the instructions above.
In a medium bowl, mix the minced onion, celery, and mayo, and reserve in the fridge.
Make the chicken salad: Once cooled, add the chicken to the mayo mixture. Season with salt and pepper to taste.
Scoop out the inside doughy part of the bagel. Then lightly toast. Layer with cheese, lettuce, tomato, and chicken salad.
Cut in half and enjoy with chips, pickle, and lemonade.