Bring water in a large pot to a boil. Add chicken breast, onion, and peppercorns to the pot. When water comes back up to a boil, reduce heat to low. Simmer for about 30 minutes. Skim off any fat appearing on the surface.
Add carrots and potatoes. Continue to simmer for another 50 minutes.
Finally, add tomatoes and salt. Simmer for another 10 minutes. Turn off stove.