Place 4 cups cold water and 2 large chicken breasts in a large saucepan.
Season the water with 3 Tbsp. Diamond Crystal or 3½ tsp. Morton kosher salt.
Bring the water to a gentle boil over medium heat.
As soon as the surface of the water starts to roll, flip the breasts over with tongs, remove the pot from heat, and cover with a tight-fitting lid.
To test the meat’s doneness, insert the probe of an instant-read thermometer into the thickest part of the chicken breast—once it registers an internal temperature of 150°, remove chicken from the water.
Let the poached chicken breasts rest on a cutting board for at least 5 minutes.
Slice chicken into ¼"-thick slices.
