In a large pot, preheat the vegetable oil and add sliced onions. Saute for a couple of minutes, until fragrant. Add water and chicken breasts. Bring to boil and add the bay leaf and salt. Lower the heat to medium-low and cook the stock for about 1 1/2 hours. (White foam-like substance will appear at the top, in the beginning, just take it off with a spoon and discard.)
Take out the chicken breast, shred it using two forks and set aside. Pour the stock into a separate container and set aside for a minute. Discard the bay leaf.
(Couple of easy ways to peel the tomatoes: 1-you can make a criss-cross cut on top of tomatoes and drop them into boiling water for just a couple of minutes and the skin will be easy to peel off. 2 - you can just use a potato peeler of the tomatoes are firm enough.)
