Place ½ of the tomatoes, half of the chilies, and half of the onion in the bottom of an Instant Pot or pressure cooker. Place chicken on top.
Sprinkle taco seasoning over chicken. Add remaining tomatoes, chilies, and onion on top of chicken.
Secure the lid on the pot. Close the pressure-release valve.
Select “Poultry” or “Manual” setting. Set the time to 8 minutes for fresh chicken breasts, or 13 minutes for frozen chicken breasts. (These times are for typical 6-8 ounce chicken breasts. If yours are larger, add a couple of minutes.)
When done, let the pressure naturally release for at least 5 minutes. After that, you can turn the valve to "vent" for quick release if you're in a hurry, or continue natural release.
Remove chicken to a cutting board. Chop with a knife or shred with two forks.
Return chicken to pot, stir, and serve.