Place chicken breasts in a pot and cover with water. Parboil until chicken is cooked, and remove. Shred chicken. In a pan, heat about 1 TBSP oil over medium high, and add in onion and tomato. Cook until onion is translucent. Add in remaining ingredients. Cook until all are incorporated and well coated with seasoning. Adjust salt as needed to taste.
Vegetarian Filling
Add about 1.5 TBSP oil to a pan and heat over medium heat. Add in all ingredients and cook together for about 7 minutes, or until potatoes are soft. Adjust salt as needed to taste.
Filling and Baking
Pour about 3 spoonful of filling on on half of your Empanada slice, and sprinkle in queso fresco or other cheese if desired. Fold the other half over it to form a triangle. Pinch the edges together to seal. Coat in egg wash (if desired), and bake at 400 F for 20 minutes. Remove from oven, cool for 5 minutes, and enjoy!