Add the avocado to a medium bowl and mash with a fork.
Add the salt, lime juice, cilantro and red onion to the mashed avocado. Stir to combine, then set aside.
Rub the chicken breasts with the olive oil, then sprinkle with the taco seasoning and gently rub the seasoning into the chicken.
Slice a slit in the middle of the chicken breasts, making sure not to cut all the way through, and fill each breast with half of the prepared guacamole.
Place the chicken breasts on a foil-lined baking sheet. Place in the oven and roast for 20 minutes.