Arrange the chicken breasts in a single layer in a medium saucepan. Add enough cold water or broth to cover the chicken by about 1 inch. Sprinkle with kosher salt, and add lemon slices, onion, garlic, or herbs if desired.
Set the pot over medium heat and bring the liquid to a gentle simmer, which typically takes about 8-12 minutes. Skim off any white froth that forms on the surface—this is normal, just chicken protein and fat.
Once the liquid starts to simmer, cover the pot and reduce the heat to low. Cook for 10 to 15 minutes, or until the chicken reaches an internal temperature of 165°F.
Remove the chicken from the poaching liquid and let it rest for 5 minutes before slicing, dicing, or shredding as desired.
If you wish to save the poaching liquid, strain it through a fine mesh sieve and refrigerate or freeze for future use as a light broth.