Boil: Combine chicken breasts, pepper, garlic 1 teaspoon of salt, onion, and oregano. Add enough water to cover the chicken. Boil covered until chicken is very tender (20 - 30 mins), adding just enough water to prevent it from drying out and burning if it becomes necessary.
Shred: Cool to room temperature. Remove chicken from the liquid (you can save it and use it as chicken broth). Shred the chicken into very small pieces.
Cook: In a skillet heat the oil over medium heat. Add the shredded chicken and cook stirring until it has heated through (about 3 minutes). Pour in the tomato sauce and parsley, and cook stirring until everything is heated. Season with salt to taste.
Serve: Serve warm as a standalone dish, or cool to room temperature before stuffing empanadas and pastelones.
If you want to save some time, use supermarket roasted chicken for the filling, shred it, and skip the step in which you boil the chicken, proceeding with the remaining steps as instructed.