Heat a pot over water over high heat until boiling.
While the water is heating, prep vegetables per the instructions above.
Cook the chicken: When the water is hot, add the chicken and cook at a simmer for about 10 minutes until the chicken has reached an internal temperature of 160 °F. Remove from water and transfer to the fridge to cool.
Once the chicken is cool, dice it into small pieces and place it back in the fridge to cool fully.
Make the chicken salad: Once cooled, mix the chicken with yogurt, mayo, lemon juice, mustard, onion, celery, tarragon, and walnuts. Season with salt and pepper to taste.
Serve: Spoon the chicken salad onto the bread and top with tomatoes and cheese. Cut in half and enjoy with carrots, hummus, and sparkling water.