In the larger pot, add half of the onion in slices. Add a pinch of salt and the boneless chicken breast. Leave on medium heat until cooked.
In a separate pot of boiling water, boil three tomatoes until soft.
In a large bowl, place the cooked chicken breast and use two forks to shred the chicken.
Using a blender on high speed, blend the chipotle peppers in adobo sauce with the two garlic cloves and boiled tomatoes. Add less chipotle peppers if you wish to make it less spicy. Blend until there are no visible chunks.
Caramelize the other half of the onion on low heat with two tablespoons of vegetable oil on a saucepan.
In a separate large saucepan, add the chipotle sauce, the shredded chicken and the caramelized onions until the sauce is evenly incorporated.